2. • Isoenzyme are multiple forms ( isomers ) of the same
enzyme, but catalyse the same reaction
• The different isoenzyme catalyses the same reaction but
differ in their primary structure and kinetic properties
• So different physical and chemical properties which
include structure, electrophoretic Km and Vmax vaules
and degree of denaturation
• Not all enzymes have isoenzyme only those which are
polymeric demonstrate isoenzyme.
Definition
3. Reason for existence of isoenzyme
• 1) Isoenzyme synthesised from different genes . Eg- Malate
dehydrogenase of cytosol different from that found in
mitochondria.
• 2) Oligomeric enzymes consisting of more than one type of subunits
eg- Lactate dehydrogenase & creatinine phosphokinase .
• 3) Enzyme may be active as a monomer or Oligomer . Eg-
Glutamate dehydrogenase
• 4) Differences in carbohydrate content of glycoprotein enzymes
may be responsible for isoenzyme . Eg – Alkaline phosphate
5. Isoenzyme of lactate
dehydrogenase (LDH)
• LDH is a tetrameric enzyme that catalysed
oxidation of L- lactate to pyruvate
• LDH has five isoenzyme :- LDH1 , LDH2 ,
LDH3, LDH4 , LDH5
• These can be separated by electrophoresis .
• Structure:- made up of 2 polypeptide subunits :- M
( muscle ) & H ( heart) type .
6.
7. Diagnostic importance of LDH:-
• Helps in diagnosis of heart and liver related disorders.
• In normal individuals, LDH2 greater than LDH1. In
case of myocardial infarction, LDH1 greater than
LDH2 and this happens between 12- 24hours after
infarction.
• LDH5- in serum – indicator of liver diseases.
• Predominant elevation of LDH2 and LDH3 in
leukemia
8. Isoenzyme of creatinine
phosphate
• Creatine kinase (CK) or creatine phosphokinase (CPK)
catalyses the inter-conversion of phospho-creatine (or
creatine phosphate) to creatine.
• CPK exists as three isoenzymes. Each isoenzyme is a
dimer composed of two subunits—M (muscle) or B
(brain) or both.
9. • CK activity greatest in striated muscle, brain and heart tissue.
The liver and erythrocytes devoid of CK activity.
• Made up of 2 type of polypeptide chains :- M muscle and H
heart type .
• 3 isoenzyme – CK1(BB), CK2(MB) , CK3(MM)
• Cardiac tissue is only the tissue which has mixed CK2 enzyme.
• CPK2 is the earliest reliable indication of myocardial
infarction.
• Normal serum levels of CK:- Females- 15- 100 U/L , Male -
10-80 U/L
10.
11. Isoenzymes of alkaline phosphate
• ALP is a monomer. The isoenzyme are due to difference
in carbohydrate content ( sialic acid residues) .
• Most imp ALP enzymes- a1- ALP , a2 heat labile, a2 heat
stable, pre B ALP , Y ALP , etc.
• Increase in a2 heat labour ALP suggests hepatitis
whereas pre B ALP indicates bone diseases.
12. Isoenzymes of Alcohol
dehydrogenase
• ADH has 2 heterodimer isoenzymes
• aB1 isoenzyme- predominant in Americans and European
and aB2 in Japanese and Chinese.
• Isomer aB2 converts alcohol to acetaldehyde more rapidly.
• Japanese and Chinese have increased sensitivity to alcohol
due to presence of aB2 isoenzymes of ADH.
