Diagnostic and Cardiac Enzymes

• Diagnostic enzymes are valuable markers in identifying tissue damage and organ dysfunction.

Isoenzymes

• These are multiple molecular forms of an enzyme catalyzing the same reaction.
• They differ in physical and chemical properties like structure, Km, Vmax, electrophoretic mobility, and susceptibility to inhibitors.
• Isoenzymes are synthesized in different tissues and sometimes within a single cell but in different organelles.
• Isoenzymes are oligomeric proteins and possess a quaternary structure with two or more subunits.
• They are products of different genes which are located in different chromosomes.
• Salivary and pancreatic amylase are examples of isoenzymes.
• Isoenzyme estimation aids in diagnosing organ dysfunction.

Cardiac Enzymes

• Blood flow stops and tissue damage occurs to heart muscles, leading to tissue death.
• Creatine kinase, CK-MB, Lactate dehydrogenase, and AST are key enzymes in cardiac disease diagnosis.

Creatine Kinase (CK)

• Three isoenzymes exist: CK1 (BB) found in the brain, CK2 (MB) specific to the heart myocardium, and CK3 (MM) found in skeletal muscle.
• CK onset occurs 3-6 hours after damage, peaking at 18-24 hours and returning to normal within 36-72 hours.
• CK levels alone are not sufficient to determine the specific cause of damage without further testing, thereby lacking specificity.
• Elevated CK levels are observed in muscular dystrophy, myopathies, poly myositis, crush injuries, fractures, and acute cerebrovascular accidents.
• The normal range for males is 15-100 U/L, while for females it is 10-80 U/L.
• Creatine's role in energy metabolism, particularly in muscle cells, is catalyzed by creatine kinase, converting it into phosphocreatine (creatine phosphate) as a rapidly available energy source.

Myoglobin And Troponins

• Myoglobin is a low molecular weight marker appearing early, but not a sensitive marker, as it is present in skeletal muscle.
• Cardiac-specific troponins T (cTnT) & I (cTnI) are regulatory proteins in myocardial contractility.
• Troponins are highly sensitive and specific indicators of cardiac tissue damage and are usually not detected under normal conditions.
• Following myocardial infarction, troponin levels increase to > twenty times the upper reference range.
• Troponins are helpful in detecting skeletal muscle injury or small myocardial infarctions below the detection limit for CK and CKMB measurements.
• FABP is twenty times more specific to cardiac muscle than myoglobin and presents at ten times higher in cardiac muscle than skeletal muscle.

Lactate Dehydrogenase (LDH)

• LDH(1), AST and Myoglobin are less specific to Myocardial Infarction because of their presence in other skeletal injuries.
• It is widely distributed in the body and helps maintain energy production and cellular function under anaerobic or low-oxygen conditions.
• Normal range - 100 - 200 IU/L
• Increased in MI, but not a specific enzyme
• Subunits - 4, types - 2 (H=Heart, M= Muscle)
• Isoenzymes - 5, named LDH 1 - LDH 5, LDH 1 – Heart(H4) ; 2 – RBC(H3M1) ; 3 – Brain(H2M2); 4 – liver(H1M3); skeletal muscle (M4).
• Normally LDH2 > LDH1, in AMI LDH1 > LDH2
• Onset - 6-12 hours Peak - 24 - 48 hours stays increased 6 – 8 days

Liver Enzymes

• Studied for diagnosis : Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), Alkaline phosphatase (ALP), Nucleotide phosphatase (NTP), Gamma glutamyl transferase (GGT)

Alanine Transaminase (ALT)

• Previously serum glutamate pyruvate transaminase (SGPT)
• Normal value for an adult is 10 to 40 U/L.
• ALT level is elevated (300 to 1000 U/L) in liver diseases (viral or toxic hepatitis), jaundice, and cirrhosis of the liver.

Aspartate Transaminase (AST)

• Formerly known as glutamate oxaloacetate transaminase (GOT).
• Plasma AST normal value for adults is 10 to 30 U/L.
• Increased AST level occurs after myocardial infarction.
• AST level starts increasing after 6 to 8 hours, peaks at 18 to 24 hours, and normal levels are reached by the 4th or 5th day.
• It is moderately elevated in liver disease.

Alkaline Phosphatase (ALP)

• Produced by osteoblasts (specialized cells for bone formation) and is associated with the calcification process.
• Normal levels - 40 – 126 IU/L.
• Moderate (2–3 times) increase - Hepatic diseases - infective hepatitis, alcoholic hepatitis, or hepatocellular carcinoma
• High levels (10-12 times of the upper limit)- extrahepatic obstruction (obstructive jaundice) caused by gallstones or carcinoma of the head of the pancreas
• Increased levels are present in children due to increased osteoblastic activity.
• Bone disorder characterized by the abnormal growth and remodeling of bone tissue, rickets, osteomalacia, osteoblastoma ,metastatic carcinoma of bone

Nucleotide Phosphatase (NTP)

• Also known as 5' nucleotidase
• Hydrolyses 5' nucleotides to corresponding nucleosides
• Elevated in hepatitis
• Reference range – 2-10 IU/L
• Highly increased in biliary obstruction

Gamma Glutamyl Transferase (GGT)

• Used for the synthesis of glutathione (Antioxidant, detoxification,etc).
• It is found in the liver, kidney, pancreas, intestinal cells, and prostate gland.
• Moderately elevated in infective hepatitis and prostate cancers.
• Clinically important because of its sensitivity to detect alcohol abuse.
• Elevated in alcoholics even when other liver function tests are within normal limits.
• Normal serum reference value - 10–30 U/L

Pancreatic Enzymes

• Amylase and Lipase are key diagnostic enzymes.

Amylase

• This enzyme splits starch to maltose.
• Activated by calcium and chloride ions.
• It is produced by the pancreas and salivary glands.
• Reference serum value of 50–120 IU/L.
• Levels increase 1000 times in acute pancreatitis.
• Values peak 5–12 hours after the event and return to normal within 2–4 days.
• Moderate increase in chronic pancreatitis, mumps (parotitis), and obstruction of pancreatic

Lipase

• It hydrolyzes triglyceride to beta monoglyceride and fatty acids and is present in the pancreatic secretion.
• Levels are highly elevated in acute pancreatitis and persist for 7–14 days.
• Lipase remains elevated longer than amylase and does not increase in mumps, making lipase estimation more advantageous over amylase.
• Moderately elevated in carcinoma of the pancreas, biliary diseases, and perforating peptic ulcers.

Enzymes in Muscle Disease

• Aldolase and Serum CPK are key markers.

Aldolase

• Used to be the enzyme of choice in the investigation of muscle diseases.
• It catalyses the interconversion of fructose-1,6-bisphosphate and triose phosphate.
• Found in high concentrations in the liver, skeletal muscle, brain, and neoplastic tissues.

Serum CPK

• The enzyme is sensitive and specific.
• Slightly elevated occasionally in neurogenic muscular atrophy.
• Raised values occur in muscular dystrophies and dermatomyositis, usually 1000 IU/L.
• Highest values are found in Duchenne muscular dystrophy (10,000 IU/L).
• The increase is most marked in the acute phase, especially in early childhood.
• It helps detect “carriers”
• Elevated in hypothyroidism due to secondary muscle disease.

Other Enzymes

• Acid Phosphatase and Cholinesterase are key enzymes.

Acid Phosphatase

• Acid phosphatase (ACP) hydrolyses phosphoric acid ester at pH between 4 and 6.
• Normal reference value - 2.5–12 U/L.
• ACP is secreted by prostate cells, RBC, platelets, and WBC.
• The prostate Isoenzyme is inactivated by tartaric acid, with a normal level of tartrate labile fraction of 1 U/L.
• The ACP value is elevated in prostate cancer and highly elevated in bone metastasis of prostate cancer.
• In these conditions, the tartrate labile isoenzyme is elevated to assist the follow-up of treatment of prostate cancers.
• Blood cells contain excess quantities of ACP and care must be taken to prevent hemolysis.

Cholinesterase (ChE)

• Acetylcholinesterase, is also known as true ChE or Type 1 ChE, acts primarily on acetylcholine.
• Present in nerve endings and RBCs.
• Newly formed RBC have high levels of ChE and gradually reduces.
• The ChE level in RBCs is proportional to the reticulocyte count (blood test for immature red blood cells).
• Organophosphorus insecticides (Parathion) irreversibly inhibit ChE in RBCs.
• Measurement of ChE level in RBCs can helps determine the amount of exposure to insecticides.

Glucose-6-Phosphate Dehydrogenase (GPD)

• Enzyme functions in the hexose monophosphate shunt pathway of glucose.
• It is mainly used for the production of NADPH and prevents peroxide formation.
• NADPH is essential for preserving RBC integrity
• Drug-induced hemolytic anemia in GPD deficient individuals.
• In GPD deficiency, reduced RBC lifespan may occur, but without disease manifestations.
• In GPD deficient individuals aspirin, mepacrine, primaquine, sulpha causes damage to RBCs.
• Primaquine stimulates peroxide formation, leading to unchecked buildup of peroxides and premature cell lysis.
• Results in drug-induced hemolytic anemia.
• Fava beans (star beans, corner beans) may also induce hemolytic anemia, called favism.

Pseudocholine Esterase

• Enzyme is Produced by Liver Cells
• It may be absent in some individuals
• Absence can be indicated by Scoline Apnoea

Alcohol Dehydrogenase (ADH)

• It consists of two heterodimer isoenzymes and is an NAD+ dependent enzyme, located in the cytosol.
• White Americans and Europeans predominantly have the αβ 1 isoenzyme.
• Japanese and Chinese (Orientals) mostly present the αβ 2.
• The isomer αβ 2 converts alcohol more rapidly to acetaldehyde.
• Accumulation of acetaldehyde causes tachycardia (increase in heart rate) and facial flushing among Orientals, which is not commonly seen in whites.
• Japanese and Chinese have sensitivity to alcohol due to the presence of αβ 2 – isoenzyme of ADH.