Pancreatic Enzymes Overview

Pancreatic Proteolytic Enzymes

• Pancreatic proteolytic enzymes are secreted in inactive forms: trypsinogen, chymotrypsinogen, and procarboxypeptidase.
• These enzymes are converted to their active forms in the duodenal lumen.
• Trypsinogen is converted to trypsin by enteropeptidase (enterokinase).
• Trypsin activates the other two enzymes, chymotrypsinogen and procarboxypeptidase, to chymotrypsin and carboxypeptidase respectively.
• Pepsinogen is another similar inactive enzyme to trypsinogen.

Pancreatic Amylase

• Digests carbohydrates.
• Cleaves polysaccharides into disaccharides.
• Salivary amylase also digests carbohydrates.

Pancreatic Lipase

• Digests fats.
• Hydrolyzes dietary triglycerides into monoglycerides (MG) and two fatty acids (2FA).

Pancreatic Aqueous Alkaline Secretion

• Pancreatic enzymes function best in a slightly alkaline environment.
• Acidic chyme emptied into the duodenum needs to be neutralized quickly.
• This allows optimal functioning of pancreatic enzymes and prevents acidic damage to the duodenal wall.
• Acidic chyme is neutralized by NaHCO3-rich fluid secreted by the pancreas into the duodenum.
• This aqueous NaHCO3 component is the largest part of pancreatic secretion.

Pancreatic Duct Secretion

• CFTR (Cystic Fibrosis Transmembrane conductance Regulator) channel is responsible for chloride (Cl-) secretion.
• Sodium (Na+) and potassium (K+) also play a role in the process.
• Net secretion of HCO3- and net absorption of H+ leads to pancreatic duct secretion.
• This secretion is different from salivary secretions, which are either isotonic or hypotonic.

Cystic Fibrosis

• Thickened secretions build up in pancreatic ducts, leading to the inability of pancreatic enzymes to reach the duodenum.
• This causes pancreatic exocrine failure in children.
• Insufficient pancreatic digestive enzymes released results in incomplete food digestion.
• The pancreas is a major source of lipase, so with pancreatic enzyme deficiency, fats are maldigested.
• Digestion of proteins and carbohydrates is less affected, however, because salivary, gastric, and small intestinal enzymes contribute towards their digestion.
• Symptoms of cystic fibrosis include:
  • Steatorrhea (excess undigested fat excreted in feces; stools are greasy, odorous, and float).
  • Poor weight gain.
• Children with cystic fibrosis may require pancreatic enzyme supplements.
• The ability to absorb fat-soluble vitamins (A, D, E, and K) is also decreased.

Regulation of Pancreatic Secretion

• Pancreatic exocrine secretion is mainly regulated by hormonal control.
• Major stimulation of pancreatic secretion occurs during the intestinal phase of digestion.
• As chyme enters the small intestine, pancreatic secretion is stimulated.
• Two major enterogastrones are released: secretin and cholecystokinin (CCK).

Secretin

• Secretin is released from S cells in the duodenum.
• Secretin stimulates the pancreas to secrete a bicarbonate-rich fluid that helps neutralize the acidic chyme entering the duodenum.

CCK

• CCK is released from I cells in the duodenum.
• CCK stimulates the pancreas to release digestive enzymes, including proteases, amylases, and lipases.
• It also stimulates gallbladder contraction and relaxation of the sphincter of Oddi, allowing bile to enter the duodenum for fat digestion.

Enzyme Secretion from Pancreatic Acinar Cells

• Gastrin, released during the cephalic and gastric phases, plays a role in enzyme secretion.
• Gastrin-releasing peptide (GRP), also known as bombesin, is important for this pathway.
• Secretin and CCK, released during the intestinal phase, also contribute to enzyme secretion.
• There is inhibitory regulation involved in this process.

Liver

• The liver is a vital organ that performs numerous functions, including:
  • Storage: Glycogen, fat, proteins, and vitamins.
  • Production of cellular fuel: Glucose and fatty acids.
  • Detoxification: Processing of harmful products ingested orally and drugs.
  • Removal: Bacteria and old red blood cells.
  • Synthesis: Plasma proteins and clotting factors (e.g. Prothrombin).
  • Activation: Vitamin D.
  • Secretion: Bile salts.
  • Excretion: Bilirubin.

Liver Structure

• Liver functional units are lobules – hexagonal tissue surrounding a central vein.
• The outer edge of the lobule has three vessels: Portal triad (hepatic artery, portal vein, and bile duct).

Bile Salts

• Bile salts are amphipathic; they have both hydrophilic and hydrophobic regions enabling them to interact with both water and fats.
• They are important in fat digestion by emulsifying fats, increasing their surface area for lipase action.
• Bile salts are reabsorbed from the small intestine and recycled into the liver, a process called the enterohepatic circulation.

Bilirubin

• Bilirubin is a yellow pigment and a waste product of heme breakdown from old red blood cells.
• It is not involved in digestion and is excreted in bile.
• Hepatocytes take up bilirubin, modify it for increased solubility, and excrete it into bile.
• Bacterial enzymes modify bilirubin in the intestines, giving feces its normal color.
• If bile secretions are blocked, fecal color will change to a more greyish color.
• Small amounts of bilirubin are reabsorbed from the intestines and excreted in urine, giving urine its typical color.

Cirrhosis

• Cirrhosis is prolonged hepatic inflammation, often associated with alcohol intake (over 60% of cases in the US).
• Damaged hepatocytes are replaced by connective tissue and collagen deposition, reducing active liver tissue and leading to chronic liver failure.
• Symptoms:
  • Jaundice.
  • Weight loss.
  • Abdominal pain.
  • Abdominal ascites (fluid retention).
• Treatment:
  • Abstinence from alcohol.
  • Diuretics.
  • Antibiotics.

This quiz covers pancreatic proteolytic enzymes, amylase, lipase, and their functions in digestion. Learn how these enzymes are activated and their roles in breaking down proteins, carbohydrates, and fats. Test your knowledge of enzyme activation and the importance of the alkaline environment in the duodenum.