Iso-enzymes
Objective
At the end of this lecture, student will be able to
• Explain Iso-enzymes
• Discuss its application in clinical diagnosis
Iso-enzymes
• The multiple forms of an enzyme catalysing the same reaction are known as iso-enzymes or isozymes, but differ in their physical and chemical properties which include the structure, electrophoretic and immunological properties, Km and Vmax values, pH optimum & degree of denaturation
• Many possible reasons are offered to explain the presence of isoenzymes in the living systems
lsoenzymes of lactate dehydrogenase (LDH)
• LDH systematic name is L-lactate-NAD+ oxidoreductase, catalyses the interconversion of lactate and pyruvate
• LDH has five distinct isoenzymes LDH1, LDH2, LDH3, LDH4 & LDH5
• They can be separated by electrophoresis
• LDHI has more + charge and fastest in electrophoretic mobility, while LDH5 is the slowest
Structure of LDH isoenzymes:
• LDH is an oligomeric (tetrameric) enzyme made up of four polypeptide subunits. Two types of subunits namely M (for muscle) and H (for heart) are produced by different genes
• M-subunit is basic while H subunit is acidic
• The isoenzymes contain either one or both the subunits giving LDHI to LDH5
Characteristic features of LDH isoenzymes
Significance of LDH:
• LDHl is found in heart muscle and is inhibited by pyruvate. Hence, pyruvate is not converted to lactate in cardiac muscle but is converted to acetyl CoA & enters citric acid cycle
• LDH5 is found in skeletal muscle and the inhibition of this enzyme by pyruvate is minimal, hence pyruvate is converted to lactate
• LDH5 has low KM (high affinity), while LDHl has high KM (low affinity) for pyruvate
• The differential catalytic activities of LDHl & LDH5 in heart and skeletal muscle, respectively are well suited for the aerobic & anaerobic conditions.
Diagnostic importance of LDH:
• Lsoenzymes of LDH have immense value in the diagnosis of heart and Iiver related disorders
• In healthy individuals, the activity of LDH2 is higher than that of LDHl in serum
• In the case of myocardial infarction, LDHI is much greater than LDH2 and this happens within 12 to 24 hours after infarction
• Increased activity of LDH5 in serum is an indicator of liver diseases
• LDH activity in the RBC is 80-100 times more than that in the serum.
Hence for estimation of LDH, serum should be totally free from hemolysis or else false positive results will be obtained
Isoenzymes of Creatine kinase (CK) or creatine phosphokinase (CPK)
• Catalyses the interconversion of phosphocreatine to creatine
• CPK exists in three isoenzyme form CPK1, CPK2, CPK3, Each is a dimer composed of two subunits, [M] muscle & [B] (brain) or both
• In healthy individuals, the isoenzyme CPK2 is almost undetectable in serum
• After Ml, within first 6 – 18 hrs, CPK2 level increase in serum
• CPK2 isoenzynre is not elevated in skeletal muscle disorders
• CPK2 is the earliest indication of myocardial infarction
Isoenzymes of alkaline phosphatase (ALP)
• As many as Six isoenzymes of ALP has been identified
• ALP is a monomer, the isoenzymes are due to the difference in the carbohydrate content
• The most important ALP isoenzymes are α1-ALP, α2-heat labile, α2-heat stable ALP, Pre-β ALP, γ-ALP etc
• ↑se α2-heat stable ALP suggests hepatitis whereas Pre-β ALP indicate bone diseases
Isoenzymes of Alcohol dehydrogenase (ADH):
• ADH has two heterodimer isoenzymes
• White Americans and Europeans, αβ1 isoenzyme is predominant whereas in Japanese and Chinese αβ2 is mostly present
• αβ2 more rapidly converts alcohol to acetaldehyde then αβ1
• Accumulation of acetaldehvde is associated with tachycardia and facial flushing among orientals which is not commonly seen in whites
• lt is believed that Japanese and Chinese have increased sensitivity to alcohol due to the presence of αβ2isoenzyme of ADH
Summary
• The multiple forms of an enzyme catalysing the same reaction are known as isoenzymes or isozymes
• LDH has five distinct isoenzymes LDH1, LDH2, LDH3, LDH4 & LDH5
• CPK exists in three isoenzyme form CPK1, CPK2, CPK3,
• The most important ALP isoenzymes are α1-ALP, α2-heat labile, α2-heat stable ALP, Pre-β ALP, γ-ALP etc
• ADH isoenzymes are αβ1 & αβ2
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