Biochemistry · Topic
Isoenzymes !
Isoenzymes carry out the same reaction but are different proteins; each tissue has its own set, so in blood they reveal which organ is damaged.
Updated 2026-11-01
Key facts
- Isoenzymes: catalyse the same reaction but differ in sequence, Km, regulation, charge and tissue of origin; they are separated by electrophoresis.
- LDH: tetramer of H (heart) and M (muscle) subunits. LDH1 (H₄) in heart, red cells and kidney; LDH5 (M₄) in liver and skeletal muscle.
- Normally LDH2 > LDH1; a “flip” to LDH1 > LDH2 suggests infarction or haemolysis.
- CK: dimer of M and B subunits. CK-MM in skeletal muscle, CK-MB in myocardium, CK-BB in brain.
- Alkaline phosphatase: liver, bone, intestinal and placental isoenzymes (the placental one is heat-stable).
In detail
- Isoenzymes come from different genes or from combining different subunits (as in LDH and CK).
- CK-MB rises about 3–6 hours after an infarct, peaks around 24 hours and returns to normal in 2–3 days: useful to detect reinfarction.
- Cardiac troponin (T or I) has now replaced CK-MB and LDH for diagnosing infarction because it is more specific.
- Total LDH rises in haemolysis, lymphoma, liver damage and infarction; its isoenzymes point to the source.
- Bone alkaline phosphatase rises during growth, Paget disease, rickets and bone metastases; liver ALP rises in cholestasis (together with GGT).
- Hexokinase (low Km, all tissues, inhibited by glucose 6-phosphate) versus glucokinase (high Km, liver and β cells, induced by insulin): another isoenzyme pair.
- Amylase has salivary and pancreatic isoforms; ALT and AST have cytosolic and mitochondrial isoforms.
- Also called
- Enzyme isoforms, LDH, CK and alkaline phosphatase
Clinical notes
- A very high total CK dominated by CK-MM points to muscle damage (rhabdomyolysis, myositis, intense exercise), not infarction.
- High alkaline phosphatase with normal GGT points to bone; with high GGT, to the biliary tree.
- The Regan placental isoenzyme can appear with some tumours (a tumour marker).
Related conditions
Related
- Enzyme kinetics (Michaelis–Menten and Lineweaver–Burk)
- Hexokinase
- Lactate
- Alanine aminotransferase (ALT, GPT)
- Aspartate aminotransferase (AST, GOT)
- Plasma proteins and electrophoresis
Test yourself
Think of the answer, then tap the question to see it.
Which LDH pattern suggests myocardial infarction?
The LDH1 > LDH2 “flipped” pattern, because the heart is rich in LDH1 (H₄).
Why is CK-MB better than troponin for detecting reinfarction?
It returns to normal in 2–3 days, while troponin stays high for 7–10 days; a fresh rise in CK-MB means a recent event.
What helps tell whether a high alkaline phosphatase comes from liver or bone?
GGT: if it is high, the ALP is hepatobiliary; if it is normal, bone is the likely source.
What do hexokinase and glucokinase share, and what sets them apart?
Both phosphorylate glucose; hexokinase has a low Km and is inhibited by its product, glucokinase has a high Km and works after meals.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: LDH isoenzyme blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Creatine phosphokinase test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Alkaline Phosphatase: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Troponin Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.