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Isoenzymes
Isoenzymes
Isoenzymes
or isozymes are multiple forms of same
enzyme that catalyze the same chemical reaction
Different
chemical and physical properties:
Electrophoretic
Kinetic
mobility
properties
Amino
acid sequence
Amino
acid composition
Lactate Dehydrogenase
 Lactate
dehydrogenase (LDH) is an enzyme present in a
wide variety of organisms
 EC
1 = oxidoreductase.
 EC
1.1 = acting on the CH-OH group of the donor.
 EC
1.1.1 = With NAD or NADP as acceptor.
 EC
1.1.1.27 = L-lactate dehydrogenase.
 Molecular
weight- 32 kD & it is tetramer
M
(A) -muscle –chromosome 11(basic)
H
(B) -heart – chromosome 12(acidic)
Lactate
dehydrogenase, reversibly converts lactate to
pyruvate, in different tissues.
LDH
consists of 5 iso-enzymes –
LDH1,LDH2,LDH3,LDH4 & LDH5
These
isoenzymes are separated by cellulose acetate
electrophoresis at pH 8.6
Normal
Serum
values:
-100 -200 U/L
CSF
- 7 -30 U/L
Urine
- 40 -100 U/L
LDH reaction
LDH isoforms
LDH isoforms
Isoenzyme name
Composition
Electrophoretic
migration
Present in
Elevated in
LDH 1
Heat resistant
( H4)
Fastest moving
Myocardium,
RBC, kidney
myocardial infarction
Kidney disease,
megaloblastic anemia
LDH2
Heat resistant
(H3M1)
Myocardium,
RBC, kidney
LDH3
(H2M2)
brain
Leukemia, malignancy
LDH4
Heat labile
(H1M3)
Lung, spleen
Pulmonary infarction
Skeletal muscle,
Liver
Skeletal muscle and liver
diseases
LDH5
Heat labile
Inhibited by urea
(M4)
Slowest moving
Clinical significance of LDH
 In
normal serum, LDH2 (H3M) predominant isoenzyme
& LDH5 is rarely seen.
 In
myocardial infarction, LDH1(H4) levels are greater
than LDH2.
 Megaloblastic
anemia (50 times upper limit of LDH 1
and LDH 2)
 Muscular
 Toxic
dystrophy, LDH5 (M4) is increased.
hepatitis with jaundice (10 times more LDH5)
Renal
disease- tubular necrosis or pyelonephritis, pulmonary
embolism LDH 3 (massive destruction of platelets)
Total
LDH is increased in neoplastic diseases.
LDH5
is increased in breast cancer, malignancies of CNS,
prostatic carcinoma.
In
leukaemias, LDH2 & LDH3 levels are increased.
In
malignant tumors of testis & ovaries, LDH2, LDH3 & LDH 5
levels are increased.
In
CSF:
Bacterial
Viral
meningitis – LDH4 and LDH5
meningitis
Metastatic
Neonatal
tumors
- LDH1
- LDH5
cases of intracranial haemorrhage
associated with seizures and hydrocephalus
Creatinine phosphokinase (CPK)

It catalyses creatine to creatine phosphate.

Normal serum value:

15-100 U/L for males & 10-80 U/L for females.

CPK consists of 3 isoenzymes.

Each isoenzyme of CK is a dimer;

Molecular weight of 40 kD.

The subunits are called B for brain (chromosome -14) &
M for muscle (chromosome -19)
Creatine Phosphokinase (CPK)



Three Iso-enzymes are separated by electrophoresis.
CPK-1
(also called CPK-BB) is found mostly in the brain & lungs.
CPK-2
(also called CPK-MB) is found mostly in the heart.
CPK-3
(also called CPK-MM) is found mostly in skeletal muscle.
Creatine phosphokinase isoenzymes
ISOENZYMES
SUB-UNIT
TISSUE
% IN SERUM
CK1
Fast moving
BB
Brain
1
CK2
2% of total
MB
Heart
5
MM
Skeletal muscle
80
CK3
Slow moving
Clinical significance of CK

CPK & heart attack:

CPK2 isoenzymes is very small, (2% of total CPK
activity) & undetectable in plasma.

In myocardial infarction (MI), CPK2 levels are
increased within 4 hrs, then falls rapidly.

Total CPK level is elevated upto 20-folds in MI.
CPK & Muscle diseases

CPK level is elevated in muscular dystrophy (5001500U/L)

CPK level is highly elevated in crush injury, fracture &
acute cerebrovascular accidents.

Estimation of total CPK is employed in muscular
dystrophies & CPK-MB isoenzyme is estimated in
myocardial infarction.
Atypical forms of CK
Two
1.
atypical isoforms.
Macro-CK (CK-macro)
Formation:
Formed
Also
by aggregation CK-MB with IgG, sometimes IgA.
formed by complexing CK-MM with lipoproteins.
Electrophoretically
Occurs
migrates between CK-MB & CK-MM.
frequently in women above 50 years.
CK-Mi (Mitochondrial CK-Isoenzyme)
Formation:
It
is present bound to the exterior surface of inner
mitochondrial membrane of muscle, liver & brain.
It
exist in dimeric form & oligomeric aggregates &
molecular weight 35,000
Electrophoretically,
migrates towards cathode & is behind
CK-MM band.
It
is not present in normal serum.
Clinical significance
It
is present in serum when there is extensive tissue
damage causing breakdown of mitochondrial & cell wall.
Its
presence in serum indicates cellular damage, seen in
malignancies.
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