Hepatic and Pancreas Enzymes PDF

HEPATIC AND PANCREAS ENZYMES Muttia Amalia FK UPN Veteran Jakarta Oktober 2024 ◦ Enzymes are protein catalysts utilized by essentially all mammalian cells in specific biochemical reactions in different organs of the body, which may also be physically located in differ...

HEPATIC AND PANCREAS ENZYMES Muttia Amalia FK UPN Veteran Jakarta Oktober 2024 ◦ Enzymes are protein catalysts utilized by essentially all mammalian cells in specific biochemical reactions in different organs of the body, which may also be physically located in different organelles and structures within a cell. ◦ Enzymes lower the activation energies of the chemical reactions that they catalyze, so as to cause greatly enhanced rates of reaction. They do not become modified in these reactions and do not affect the equilibrium between reactants and products in the reaction. The lock-and-key model of substrate binding to the enzyme active site. The enzyme exhibits preformed steric and electronic complementarity to the shape and charge distribution of the substrate. No shape changes or electronic redistributions in the enzyme or the substrate are necessary for optimal binding The induced-fit model of substrate binding to the enzyme active site. The induced-fit model postulates an initial weak, flexible interaction of the substrate with groups in the enzyme’s substrate (ES) binding site. This is sufficient to trigger a conformational rearrangement of the enzyme’s surface that exposes additional ligand binding groups that enhance the binding affinity of the substrate for the enzyme SPECIFIC ENZYMES ◦ Numerous enzymes are clinically useful in recognition and monitoring of particular disease processes. ◦ In all but a few circumstances, abnormal conditions in specific tissues are recognized by elevations of one or more enzymatic activities or enzyme concentrations. ◦ For example, elevations of the activity of serum CK suggest the presence of muscle disease. ◦ The reason for elevation of specific enzymes in blood and other body fluids as the result of disease processes in specific tissues is not completely understood. ◦ One explanation for this occurrence is that disease processes that cause cell injury or death result in damage to the cell membrane, leading to the release of specific intracellular enzymes into tissue spaces and the microvasculature, causing increased enzymatic activity in serum or other body fluids. Liver, the largest gland in the body, a spongy mass of wedge-shaped lobes that has many metabolic and secretory functions. The liver secretes bile, a digestive fluid; metabolizes proteins, carbohydrates, and fats; stores glycogen, vitamins, and other substances; synthesizes blood-clotting factors; removes wastes and toxic matter from the blood; regulates blood volume; and destroys old red blood cells. Hepatic enzymes ◦ The liver cells synthesize a number of enzymes. ◦ As blood flows through the liver, both from the portal vein and from the hepatic artery, the cells and enzymes are filtered. ◦ Nutrients entering the liver from the intestine are modified into forms that are usable by the body cells or are stored for future use. ◦ Fats are converted into fatty acids and then into carbohydrates or ketone bodies and transported by the blood to the tissues, where they are further metabolized. ◦ Sugars are converted into glycogen, which remains stored in the liver until it is needed for energy production; it is then reconverted into glucose and released into the bloodstream. ◦ The liver manufactures blood serum proteins, including albumin and several clotting factors, and supplies them to the blood. ◦ The liver also metabolizes nitrogenous waste products and detoxifies poisonous substances, preparing them for elimination in the urine or feces. NORMAL LIVER FUNCTION ◦ The liver is the largest and most complex organ of the gastrointestinal tract. ◦ Overall, it comprises three systems: ◦ First, the biochemical hepatocytic system, which is responsible for the vast majority of all metabolic activities in the body  ◦ including protein synthesis; aerobic and anaerobic metabolism of glucose and other sugars; ◦ glycogen synthesis and breakdown; ◦ amino acid and nucleic acid metabolism; amino acid and dicarboxylic acid interconversions via transaminases (aminotransferases); lipoprotein synthesis and metabolism; ◦ xenobiotic metabolism (e.g., drug metabolism), usually involving the cytochrome P450 oxidation system; storage of iron and vitamins such as A, D, and B12; ◦ and synthesis of hormones such as angiotensinogen, insulin-like growth factor I, and triiodothyronine. It is also the site of clearance of many other hormones such as insulin, parathyroid hormone, estrogens, and cortisol. Uniquely, the liver is the site of metabolism of ammonia to urea. ◦ Albumin in the body is synthesized in the liver, as are all coagulation factor proteins, with the exception of von Willebrand factor, which is synthesized in endothelial cells and megakaryocytes. Patients with liver disease may have signs or symptoms related to disturbance of any of the above functions. ◦ The second major hepatic system is the hepatobiliary system, which is concerned with the metabolism of bilirubin, a process that involves transport of bilirubin into the hepatocyte and its conjugation to glucuronic acid and its secretion into bile canaliculi and the enterohepatic system. ◦ Last is the reticuloendothelial system—that is, Kupffer cells. These are a form of macrophage involved (a) with the immune system, including being a major site of defense against intestinal bacteria and the primary location for removal of antigen–antibody complexes from the circulation, and (b) with the breakdown of hemoglobin from dead erythrocytes, giving rise to bilirubin, which, together with bilirubin from the spleen, enters the hepatocyte Bilirubin ◦ Bilirubin is an important metabolite of heme (ferroprotoporphyrin IX), a coordination complex coordinating iron in various proteins. It is a potentially toxic substance. However, the body has developed mechanisms for its safe detoxification and disposition. Bilirubin and its metabolites also provide a distinctive yellow color to bile and stool and, to a lesser degree, urine. This topic summarizes the mechanism of heme metabolism and bilirubin synthesis. ◦ Bilirubin is derived from 2 main sources. Roughly 80% of bilirubin is made from the breakdown of hemoglobin in senescent red blood cells and prematurely destroyed erythroid cells in the bone marrow. The remainder originates from the turnover of various heme-containing proteins found in other tissues, primarily the liver and muscles. These proteins include myoglobin, cytochromes, catalase, peroxidase, and tryptophan pyrrolase. Approximately 4 mg/kg body weight of bilirubin is produced daily. Cellular Heme Metabolism ◦ Heme is a ring of 4 pyrroles joined by carbon bridges and a central iron atom. ◦ Bilirubin is generated by a 2-stage sequential catalytic degradation reaction that primarily occurs in the reticuloendothelial system's cells, notably the spleen. Other cells include phagocytes and the Kupffer cells of the liver. ◦ These cells take up the heme, and enzyme heme oxygenase acts on them. The enzyme liberates the chelated iron by catalyzing the oxidation of the alpha-carbon bridge. This reaction produces an equimolar amount of carbon monoxide, which is excreted by the lungs and leads to the formation of the green pigment biliverdin. This green pigment is acted upon further by the nicotinamide adenine dinucleotide phosphate (NADPH) dependent enzyme, biliverdin reductase. This process releases an orange-yellow pigment known as bilirubin. Heme oxygenase, as mentioned above, is present in high concentrations in the liver's Kupffer cells and the reticuloendothelial system's cells. Heme oxygenase is the rate-limiting factor in bilirubin production. ◦ The final structure is highly compacted by hydrogen bonding rendering the molecule essentially insoluble in aqueous solutions at neutral pH. The fully bonded structure of bilirubin is designated as bilirubin IX-alpha-ZZ. Bilirubin, insoluble in an aqueous solution, is carried in circulation bound to albumin, a reversible and covalent type of bonding. Metabolism of Bilirubin ◦ Albumin binding: Once bilirubin is released into the plasma, it is taken up by albumin, a transporter throughout the body. The binding affinity for albumin to bilirubin is extremely high, and under ideal conditions, no free (non-albumin bound) unconjugated bilirubin is seen in the plasma. To a lesser degree, especially in states of hypoalbuminemia, binding also occurs with high-density lipoprotein. The binding of albumin limits the escape of bilirubin from the vascular space, minimizes glomerular filtration, and prevents its precipitation and deposition in tissues. ◦ When the albumin-bilirubin complex reaches the liver, the highly permeable hepatic circulation allows the complex to reach the sinusoidal surface of the hepatocyte. This allows the pigment to disassociate from the albumin and enter the liver. This process is relatively inefficient, with the first pass clearance of bilirubin being approximately 20%. This inefficient process allows for always having the ability to measure a concentration of unconjugated bilirubin bound to albumin in the venous circulation. The binding of albumin to bilirubin is reversible. ◦ Hepatic transport mechanisms: Bilirubin is taken up into the hepatocytes from the liver sinusoids by 2 different mechanisms: passive diffusion and receptor-mediated endocytosis. ◦ The passive diffusion process is not energy-consuming and, as a result, follows a concentration gradient, making the flow bi-directional. ◦ Active transporter uptake of unconjugated bilirubin from the hepatic sinusoids is mediated by carrier proteins that are not well understood. Most of the unconjugated bilirubin entering the hepatocytes is extracted in the periportal region. A fraction of conjugated and unconjugated bilirubin within the hepatocyte is transported back into the sinusoidal space, and this fraction is once again taken up downstream to the sinusoidal flow. ◦ The 1A mediates the uptake, and 1B members of the organic anion transporting polypeptide family (OATP). These polypeptides are encoded by the genes: SLCO1B1 and SLCO1B3. Conjugated bilirubin that escapes reuptake into the hepatocyte is excreted in the urine. Bilirubin binding to glutathione S-transferases increases net uptake and minimizes the efflux of internalized bilirubin. Hepatocyte Conjugation ◦ Conjugation is mandatory to render bilirubin aqueous soluble and facilitate its secretion across the canalicular membrane and excretion into bile. Bilirubin is conjugated within the hepatocyte to glucuronic acid by a family of enzymes termed uridine-diphosphoglucuronic glucuronosyltransferase (UDPGT). The process of glucuronidation is 1 of the many crucial detoxification mechanisms of the human body. ◦ Many different isoforms of UDPGT exist, but the physiologically important isoform in bilirubin glucuronidation is UDPGT1A1. The enzyme esterifies 2 glucuronide moieties to bilirubin's propionic acid side chains. Under normal conditions, bilirubin diglucuronide is the predominant molecule synthesized. However, if the conjugation system is overwhelmed under conditions of excessive bilirubin synthesis, most bilirubin may be conjugated as bilirubin monoglucuronide. ◦ The ratio of mono-conjugated to the dis-conjugated pigment in bile is 1:4. Conjugation of bilirubin to the water-soluble form involves the disruption of the hydrogen bonds, an essential process for its elimination by the liver and kidney. This is achieved by glucuronic acid conjugating bilirubin's propionic acid side chains ◦ Excretion of conjugated bile: Conjugated bilirubin and other substances destined to be excreted in bile are actively transported across the bile canalicular membrane of the hepatocyte. The concentration gradient is very high and can reach 1:1000. At least 4 known canalicular transporters participate in the excretion of conjugated bilirubin. ◦ However, the multidrug resistance-associated protein 2 (MRP2) appears to play the dominant role in the canalicular secretion of conjugated bilirubin. A portion of conjugated bilirubin is transported into the sinusoids and portal circulation by MRP3, which can undergo hepatocyte reuptake via the sinusoidal proteins, organic anion transport protein 1B1 and 1B3 (OATP1B1 and OATP1B3). Thus, some conjugated and unconjugated bilirubin may escape the hepatocyte cytosol into the plasma, which binds to albumin and gets transported around the body. However, only conjugated bilirubin can enter the bile. ◦ The conjugated bilirubin is then secreted into canalicular bile and drains into the small intestine. The rate- limiting step in bilirubin throughput is the hepatic excretory capacity of conjugated bilirubin. Part of the conjugated bilirubin may accumulate in serum when the hepatic excretion of the conjugated bilirubin is impaired, as in prolonged biliary obstruction or intrahepatic cholestasis. This conjugated bilirubin fraction gets covalently bound to albumin and is called delta bilirubin, delta fraction, or biliprotein. As the delta bilirubin is bound to albumin, its clearance from serum takes about 12-14 days (equivalent to the half-life of albumin) in contrast to the usual 2 to 4 hours (half-life of bilirubin). ◦ The conjugation process alters the physicochemical properties of bilirubin, giving it many special properties. Most importantly, it makes the molecule water-soluble, allowing it to be transported in bile without a protein carrier. Conjugation also increases the size of the molecule. Conjugation prevents bilirubin from passively being reabsorbed by the intestinal mucosa due to its hydrophilicity and large molecular size. Thus, conjugation works to promote the elimination of potentially toxic metabolic waste products. Furthermore, conjugation modestly decreases the affinity of bilirubin for albumin. ◦ Degradation in the digestive tract: Conjugated bilirubin is not reabsorbed from the proximal intestine as mentioned above; in comparison, unconjugated bilirubin is partially reabsorbed across the lipid membrane of the small intestinal epithelium and undergoes enterohepatic circulation. Within the proximal small intestine, there is no additional bilirubin metabolism, and very little deconjugation occurs. In stark contrast, when the conjugated bilirubin reaches the distal ileum and colon, it is rapidly reduced and deconjugated by colonic flora to a series of molecules termed urobilinogen. The major urobilinoids seen in stool are known as urobilinogen and stercobilinogen, the nature and relative proportion of which depends on the presence and composition of the gut bacterial flora. These substances are colorless but turn orange-yellow after oxidation to urobilin, giving stool its distinctive color. ◦ Normal bilirubin levels vary by age and type of bilirubin: ◦ Total bilirubin: For adults over 18, the normal level is up to 1.2 milligrams per deciliter (mg/dL). For children under 18, the normal level is 1 mg/dL. For newborns, the normal level can range from 1.0 to 12.0 mg/dL. ◦ Direct (conjugated) bilirubin: The normal level is less than 0.3 mg/dL. ◦ Bilirubin is a substance found in bile, a fluid produced in the liver. A bilirubin test measures the amount of bilirubin in the blood and can help diagnose or monitor liver or red blood cell problems. ◦ Some other things to know about bilirubin levels include: ◦ Men tend to have slightly higher bilirubin levels than women. ◦ Black people tend to have lower bilirubin levels than people of other races. ◦ Adults with jaundice usually have bilirubin levels greater than 2.5 mg/dL. ◦ In an otherwise healthy newborn, bilirubin levels greater than 15 mg/dL may cause problems. Measurement of Serum Bilirubin ◦ Serum bilirubin is measured spectrophotometrically when the molecule reacts with diazo reagents, causing the breakdown of the tetrapyrrole into 2 azodipyrroles. This reaction is termed as the “Van den Bergh.” ◦ Unconjugated bilirubin reacts slowly with the diazo reagent as the central carbon bridge of bilirubin is buried within the hydrogen bonds. In contrast, conjugated bilirubin lacks these hydrogen bonds, and the reaction occurs rapidly even without accelerators. Adding accelerators such as caffeine or methanol disrupts the hydrogen bonds, and the reaction is quickly completed yielding the total bilirubin value. ◦ Unconjugated bilirubin is measured by subtracting the direct-reacting fraction from total bilirubin. Potential sources of error include plasma lipids, drugs such as propranolol, and several other endogenous substances. These interfere with the diazo assay and can potentially produce an unreliable result. The Normal bilirubin levels are typically: Total Bilirubin: 0.3 to 1.2 mg/dL, Direct Bilirubin: 0.0 to 0.3 mg/dL, Indirect Bilirubin: 0.2 to 0.8 mg/dL. Abnormal levels of bilirubin may indicate issues with the liver or bile ducts, or hemolytic anemia, which is the breakdown of red blood cells. This test is often used to identify and evaluate liver disorders such as hepatitis, cirrhosis, or jaundice. TESTS OF LIVER INJURY PLASMA ENZYME LEVELS ◦ As metabolically complex cells, hepatocytes contain high levels of a number of enzymes. ◦ With liver injury, these enzymes may leak into plasma and can be useful for diagnosis and monitoring of liver injury. Cellular Locations of Enzymes ◦ Within the hepatocyte, the commonly measured enzymes are found in specific locations; the type of liver injury will determine the pattern of enzyme change. ◦ Cytoplasmic enzymes include lactate dehydrogenase (LD), aspartate aminotransferase (AST), and alanine aminotransferase (ALT). ◦ Mitochondrial enzymes, such as the mitochondrial isoenzyme of AST, are released with mitochondrial damage. ◦ Canalicular enzymes, such as alkaline phosphatase and γ-glutamyl transferase (GGT), are increased by obstructive processes. Location of hepatocellular enzymes. ◦ The major diagnostic hepatocellular enzymes are located at various sites in the hepatocyte, giving rise to different patterns of enzyme release with different causes of injury. ◦ Alanine aminotransferase (ALT) and the cytoplasmic isoenzyme of aspartate aminotransferase (ASTc) are found primarily in the cytosol. With membrane injury as in viral or chemically induced hepatitis, these enzymes are released and enter the sinusoids, raising plasma AST and ALT activities. ◦ Mitochondrial aspartate aminotransferase (ASTm) is released primarily with mitochondrial injury, as caused by ethanol as in alcoholic hepatitis. ◦ Alkaline phosphatase (ALP) and γ-glutamyl transferase (GGT) are found primarily on the canalicular surface of the hepatocyte. Bile acids accumulate in cholestasis and dissolve membrane fragments, releasing bound enzymes into plasma. GGT is also found in the microsomes; microsomal enzyme- inducing drugs, such as phenobarbital and Dilantin, can also increase GGT synthesis and raise plasma GGT activity Aminotransferases (Transaminases) ◦ Two diagnostically very useful enzymes in this category are AST or aspartate amino transferase, also known as serum glutamate oxaloacetate transaminase, and ALT or alanine amino transferase, formerly called serum glutamate pyruvate transaminase. These enzymes catalyze reversibly the transfer of an amino group of AST or ALT to α-ketoglutarate to yield glutamate plus the corresponding ketoacid of the starting amino acid (i.e., oxaloacetate or pyruvate, respectively). Both enzymes require pyridoxal phosphate (vitamin B6) as a cofactor. ◦ AST and ALT have respective blood half-lives of 17 and 47 hours, respectively, and have upper reference range limits of around 40 IU/L. AST is both intramitochondrial and extramitochondrial, but ALT is completely extramitochondrial. Mitochondrial AST isoenzyme has a half-life of 87 hours. AST is ubiquitously distributed in the body tissues, including the heart and muscle, whereas ALT is found primarily in the liver, although significant amounts are also present in the kidney. ◦ Total cytoplasmic AST is present in highest activity in hepatocytes, with a cell AST level approximately 7000 times that in plasma. ◦ ALT is also present in highest activity in hepatocytes, with a cell ALT level approximately 3000 times that in plasma. With pyridoxine deficiency, hepatic synthesis of ALT is impaired; a similar phenomenon occurs in hepatic fibrosis and cirrhosis. ◦ The enzyme changes seen in hepatic injury can be readily explained by differing hepatic activity levels and half-lives of enzymes. With most forms of acute hepatocellular injury, such as hepatitis, AST will be higher than ALT initially because of the higher activity of AST in hepatocytes. Within 24 to 48 hours, particularly if ongoing damage occurs, ALT will become higher than AST, based on its longer half-life. ◦ In chronic hepatocyte injury, mainly in cirrhosis, ALT is more commonly elevated than AST; however, as fibrosis progresses, ALT activities typically decline, and the ratio of AST to ALT gradually increases, so by the time cirrhosis is present, AST is often higher than ALT. ◦ However, in end-stage cirrhosis, the levels of both enzymes generally are not elevated and may be low as the result of massive tissue destruction. In acute fulminant hepatic failure, the serum levels of both aminotransferases are markedly increased and are such that the AST/ALT ratio is often significantly greater than 1 Alkaline Phosphatase ◦ ALP is present in a number of tissues, including liver, bone, kidney, intestine, and placenta, each of which contains distinct isozymes that can be separated from one another by electrophoresis. ◦ Total ALP in serum is mainly present in the unbound form and, to a lesser extent, is complexed with lipoproteins or rarely with Igs. ◦ ALP in the liver, which has a half-life of about 3 days, is a hepatocytic enzyme that is found on the canalicular surface and is therefore a marker for biliary dysfunction. ◦ The bone isozyme is particularly heat labile, allowing it to be distinguished from the other major forms. ◦ In addition, small intestinal and placental ALP is antigenically distinct from liver, bone, and kidney ALP. The bulk of ALP in the serum of normal patients is made up of liver and bone ALP. ◦ In obstruction of the biliary tract by stones in the ducts or ductules, or by infectious processes resulting in ascending cholangitis, or by spaceoccupying lesions, biliary tract ALP rises rapidly to values sometimes in excess of 10 times the upper limit of normal. The reasons for this increase probably include a combination of increased synthesis and decreased excretion of ALP. ◦ In obstructive cholestasis, ALP most commonly rises to twice the upper limit of normal or greater, roughly paralleling the rate of rise in serum bilirubin. If obstruction is partial, ALP usually increases as much as with complete obstruction, often out of proportion to the increase in conjugated bilirubin (dissociated jaundice). ◦ Passive congestion of the liver can occasionally result in moderate ALP elevations, more so than abnormal bilirubin levels. ◦ ALP is also moderately elevated in most instances of jaundice resulting from hepatic injury. When the resulting cholestasis is relieved, serum ALP levels fall to normal more slowly than bilirubin ◦ A high molecular weight ALP appears in serum in cholestasis. This ALP is attached to fragments of canalicular membrane. Bile salts solubilize the enzymes from the sinusoidal and canalicular membranes. In serum, the membrane-bound enzymes aggregate with lipids and lipoproteins. ◦ Intestinal ALP is increased in a variety of disorders of the intestinal tract and in cirrhosis. Serum intestinal ALP is detected in more than 80% of cirrhotic patients as compared with 10% of normal controls. Measurement of this enzyme activity was suggested as one method of discriminating intrahepatic from extrahepatic jaundice, because intestinal ALP may be absent in extrahepatic obstruction, but it lacks adequate sensitivity and specificity. γ-Glutamyl Transferase ◦ This enzyme regulates the transport of amino acids across cell membranes by catalyzing the transfer of a glutamyl group from glutathione to a free amino acid. Its major use is to discriminate the source of elevated ALP (i.e., if ALP is elevated and GGT is correspondingly elevated, then the source of the elevated ALP is most likely the biliary tract). ◦ The highest values, often greater than 10 times the upper limit of normal, may be found in chronic cholestasis due to primary biliary cirrhosis or sclerosing cholangitis. This enzyme is also elevated in about 60% to 70% of those who chronically abuse alcohol, with a rough correlation between the amount of alcohol intake and GGT activity. Levels often decline slowly with abstention from alcohol and remain elevated for at least 1 month after abstinence begins. ◦ GGT has a half-life of 10 days, but, in recovery from alcohol abuse, the half-life may be as long as 28 days. It tends to be higher in obstructive disorders and with space-occupying lesions in the liver than with hepatocyte injury. What causes elevated liver enzymes? ◦ Common causes for elevated liver enzymes include: ◦ Certain medications, such as cholesterol-lowering drugs (statins) and acetaminophen. ◦ Fatty liver disease, including alcohol-related and non-alcohol-related conditions. ◦ Hemochromatosis. ◦ Hepatitis A, hepatitis B, hepatitis C, alcoholic hepatitis and autoimmune hepatitis. ◦ Herbal supplements and vitamin supplements, like chaparral, comfrey tea, iron and vitamin A. ◦ Other causes of elevated liver enzymes include: ◦ Alpha-1 antitrypsin deficiency. ◦ Cancer. ◦ Celiac disease. ◦ Cirrhosis of the liver. ◦ Hemolysis. ◦ Metabolic syndrome. ◦ Muscle conditions, like polymyositis. ◦ Thyroid disease. ◦ Wilson disease. ◦ Primary sclerosing cholangitis. ◦ Primary biliary cirrhosis. What are the symptoms of elevated liver enzymes? ◦ Most people with elevated liver enzymes don’t have symptoms. If liver damage is the cause of elevated liver enzymes, you may have symptoms such as: ◦ Abdominal (stomach) pain. ◦ Dark urine (pee). ◦ Fatigue (feeling tired). ◦ Itching. ◦ Jaundice (yellowing of your skin or eyes). ◦ Light-colored stools (poop). ◦ Loss of appetite. ◦ Nausea and vomiting. Pancreas Anatomically, the pancreas is divided into head, body, and tail. The pancreatic parenchyma has a lobular structure and contains numerous secretory vesicles, which make up 80– 85% of the organ’s mass. The discharge ducts are very important for the functioning of the pancreas. Each bubble has an outgoing wire that connects to the others and connects to the main duct. The main duct is the pancreatic duct, which begins in the tail of the pancreas, runs the entire length of the organ, and eventually enters the duodenum through the greater papilla (Vatera). Apart from it, there is also the accessory pancreatic duct, which in about 70% of people connects to the pancreatic duct, and finally, the substance secreted by the pancreas, transported through both ducts, goes to the so-called greater duodenal papilla. ◦ The pancreas has two essential and very important functions in the body: endocrine (production of hormones that regulate blood sugar levels and glandular secretion) and exocrine (the function of the digestive gland). ◦ Endocrine activity is performed by the Langerhans islets and involves the production of hormones such as insulin, proinsulin, amylin, C-peptide, somatostatin, pancreatic polypeptide (PP), and glucagon. Insulin helps to lower blood sugar, and glucagon causes blood sugar to rise. ◦ On the other hand, the exocrine activity consists of the production of enzymes that are part of the iso- osmotic, alkaline pancreatic juice and support the digestion of food in the intestines. The intravesical cells produce the enzymatic components of the juice, which is led into the duodenum through the pancreatic ducts. In addition, mucus is secreted in the pancreatic ducts through goblet cells. The composition of pancreatic juice includes enzymes that digest proteins, fats, carbohydrates, and nucleic acids, as well as electrolytes and a small amount of mucus. ◦ Two of the population of cells in the pancreatic parenchyma make up its digestive enzymes: ◦ Ductal cells: Mainly responsible for production of bicarbonate (HCO3), which acts to neutralize the acidity of the stomach chyme entering duodenum through the pylorus. Ductal cells of the pancreas are stimulated by the hormone secretin to produce their bicarbonate-rich secretions, in what is in essence a bio-feedback mechanism; highly acidic stomach chyme entering the duodenum stimulates duodenal cells called "S cells" to produce the hormone secretin and release to the bloodstream. Secretin having entered the blood eventually comes into contact with the pancreatic ductal cells, stimulating them to produce their bicarbonate-rich juice. Secretin also inhibits production of gastrin by "G cells", and also stimulates acinar cells of the pancreas to produce their pancreatic enzyme. ◦ Acinar cells: Mainly responsible for production of the inactive pancreatic enzymes (zymogens) that, once present in the small bowel, become activated and perform their major digestive functions by breaking down proteins, fat, and DNA/RNA. Acinar cells are stimulated by cholecystokinin (CCK), which is a hormone/neurotransmitter produced by the intestinal cells (I cells) in the duodenum. CCK stimulates production of the pancreatic zymogens. ◦ Pancreatic juice, composed of the secretions of both ductal and acinar cells, contains the following digestive enzymes: ◦ Trypsinogen, which is an inactive(zymogenic) protease that, once activated in the duodenum into trypsin, breaks down proteins at the basic amino acids. Trypsinogen is activated via the duodenal enzyme enterokinase into its active form trypsin. ◦ Chymotrypsinogen, which is an inactive (zymogenic) protease that, once activated by duodenal enterokinase, turns into chymotrypsin and breaks down proteins at their aromatic amino acids. Chymotrypsinogen can also be activated by trypsin. ◦ Carboxypeptidase, which is a protease that takes off the terminal amino acid group from a protein ◦ Several elastases that degrade the protein elastin and some other proteins ◦ Pancreatic lipase that degrades triglycerides into two fatty acids and a monoglyceride ◦ Sterol esterase ◦ Phospholipase ◦ Several nucleases that degrade nucleic acids, like DNAase and RNAase ◦ Pancreatic amylase that breaks down starch and glycogen which are alpha-linked glucose polymers. Humans lack the cellulases to digest the carbohydrate cellulose which is a beta-linked glucose polymer. ◦ The pancreas's exocrine function owes part of its notable reliability to biofeedback mechanisms controlling secretion of the juice. The following significant pancreatic biofeedback mechanisms are essential to the maintenance of pancreatic juice balance/production: ◦ Secretin, a hormone produced by the duodenal "S cells" in response to the stomach chyme containing high hydrogen atom concentration (high acidity), is released into the blood stream; upon return to the digestive tract, secretion decreases gastric emptying, increases secretion of the pancreatic ductal cells, as well as stimulating pancreatic acinar cells to release their zymogenic juice. ◦ Cholecystokinin (CCK) is a unique peptide released by the duodenal "I cells" in response to chyme containing high fat or protein content. Unlike secretin, which is an endocrine hormone, CCK actually works via stimulation of a neuronal circuit, the end-result of which is stimulation of the acinar cells to release their content. CCK also increases gallbladder contraction, resulting in bile squeezed into the cystic duct, common bile duct and eventually the duodenum. Bile of course helps absorption of the fat by emulsifying it, increasing its absorptive surface. Bile is made by the liver, but is stored in the gallbladder. ◦ Gastric inhibitory peptide (GIP) is produced by the mucosal duodenal cells in response to chyme containing high amounts of carbohydrate, proteins, and fatty acids. Main function of GIP is to decrease gastric emptying. ◦ Somatostatin is a hormone produced by the mucosal cells of the duodenum and also the "delta cells" of the pancreas. Somatostatin has a major inhibitory effect, including on pancreatic production Amylase ◦ Amylase in serum and urine is stable for 1 week at ambient temperature and for at least 6 months under refrigeration in well-sealed containers. ◦ Plasma specimens that have been anticoagulated with citrate or oxalate should be avoided for amylase determination because amylase is a calcium-containing enzyme. Heparinized plasma specimens do not interfere with the amylase assay. ◦ Diagnosis is confirmed by detection of elevated serum amylase three-fold above normal. It peaks in 20 to 30 hours, often at 10 to 20 times the upper reference limit. Amylase returns to normal in 48 to 72 hours. Elevated values persisting longer than this suggest continuing necrosis or possible pseudocyst formation. ◦ Amylase continues to be a first-line test for acute pancreatitis in current clinical practice despite certain problematic issues. Serum amylase has poor sensitivity for pancreatitis; it is not increased in about 20% of patients with pancreatitis. Lipase ◦ The pancreas is the major and primary source of serum lipase. ◦ Human pancreatic lipase is a glycoprotein with a molecular weight of 45 kDa. Lipase is not present in the salivary glands. ◦ Lipases are defined as enzymes that hydrolyze preferentially glycerol esters of long-chain fatty acids at the carbon 1 and 3 ester bonds, producing 2 moles of fatty acid and 1 mole of β-monoglyceride per mole of triglyceride. After isomerization, the third fatty acid can be split off at a slower rate. Lipolysis increases in proportion to the surface area of the lipid droplets, and the absence of bile salts in duodenal fluid with resultant lack of emulsification renders lipase ineffective. ◦ Serum lipase has been described as a better first-line test for diagnosis of acute pancreatitis than serum amylase. It has a sensitivity and specificity of 92% and 91%, respectively. Serum lipase increases in 4 to 8 hours and remains elevated for 8 to 14 days. Increased lipase activity rarely lasts longer than 14 days; prolonged increases suggest a poor prognosis or the presence of a pancreatic cyst. Hyperglycemia and elevated bilirubin concentrations may be present, and leukocytosis is frequently reported. Trypsinogen ◦ Trypsin is produced in the exocrine pancreas as two proenzymes, known as trypsinogen 1 and trypsinogen 2. ◦ These proenzymes are activated in the duodenum by an enterokinase that yields trypsin 1 and trypsin 2, respectively. Trypsin present within the peripheral circulation is inactivated by complexing with α-2- macroglobulin or α-1-antitrypsin (AAT). ◦ Trypsin, unlike amylase, is produced solely by the pancreatic acinar cells and therefore is a specific indicator of pancreatic damage. Premature activation of the proenzyme to active trypsin within the pancreatic parenchyma is thought to be a key mechanism in the development of acute pancreatitis. Currently, levels of all forms of trypsin are determined by specific immunoassays. Measurement of Exocrine Pancreatic Secretion in Humans ◦ Various measurement strategies have been developed to determine secretory function both for understanding normal physiology and determining the effects of disease states of the pancreas on secretory function. These measurements of secretory function are divided into two groups: the direct measurements and the indirect measurements. ◦ Direct measurements of pancreatic secretory function involve collection of pancreatic secretions in the duodenum either without stimulation of the pancreas or after intravenous administration of a secretagogue or a combination of secretagogues combined with measurements of the ions and digestive enzymes in the secretions. Indirect measurements of pancreatic secretory function include the measurement of pancreatic enzymes in duodenal samples after nutrient ingestion, the measurement of products of digestive enzyme action on ingested substrates, and the measurement of pancreatic enzymes in the stool. ◦ Which measurement to use depends on the physiologic or clinical question under consideration. ◦ A key point that has to be considered for any measurement strategy is that the exocrine pancreas has a large functional reserve. That is, the capacity for digestion is about 10 times of what is needed for complete digestion and absorption of a meal. ◦ Nutrient loss in the stool does not occur unless the functional capacity of the exocrine pancreas is less than 5% to 10% of normal as measured by CCK-stimulation of digestive enzyme secretion into the duodenum (a direct measurement technique). ◦ Thus, the indirect measurements that depend on the conversion of an ingested substrate to a measurable product will be insensitive to the changes in function that may occur in a disease state. Thus, the direct measurement of duodenal digestive enzymes, ions and water after the intravenous administration of pancreatic secretagogues provides the greatest sensitivity. ◦ More recently, a direct measurement method has been developed using endoscopy, and a short collection period has been described. An endoscopy which contains aspirating channels is passed into the duodenum. Then, secretin, CCK or the combination of the two is administered intravenously, and pancreatic secretions are collected via the endoscope tip with its aspiration channel positioned in the duodenum. ◦ The classic indirect measurement of pancreatic function with ingestion of a meal is the “Lundh Test Meal.” In the original description, the subject ingests a 300-mL liquid test meal composed of dried milk, vegetable oil and dextrose (6% fat, 5% protein and 15% carbohydrate). Samples are aspirated from the duodenum at intervals for measurement of digestive enzymes. The results of this type of measurement are dependent on the entire physiologic system, including the various sensory inputs during the different phases of the meal, the neurohumoral transmission systems and the pancreatic responses to the neurohumoral system. Thus, pancreatic enzyme secretory responses will be influenced by disorders of sensory organs, mucosal diseases of the upper intestine and alterations in the anatomy of the upper gastrointestinal tract. Comparisons of the results of the Lundh test meal (or variations of this meal) with those of direct measurement of pancreatic function can be used to show the influence of these factors on the pancreatic response Terimakasih