Iso-enzymes

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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