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[Biochemistry](https://durgical.com/en/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

- [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/)
- [Acute pancreatitis](https://durgical.com/en/conditions/acute-pancreatitis/)
- [Viral hepatitis](https://durgical.com/en/conditions/viral-hepatitis/)

## Related

- [Enzyme kinetics (Michaelis–Menten and Lineweaver–Burk)](https://durgical.com/en/biochemistry/topics/enzyme-kinetics/)
- [Hexokinase](https://durgical.com/en/biochemistry/enzymes/hexokinase/)
- [Lactate](https://durgical.com/en/biochemistry/molecules/lactate/)
- [Alanine aminotransferase (ALT, GPT)](https://durgical.com/en/biochemistry/enzymes/alanine-aminotransferase/)
- [Aspartate aminotransferase (AST, GOT)](https://durgical.com/en/biochemistry/enzymes/aspartate-aminotransferase/)
- [Plasma proteins and electrophoresis](https://durgical.com/en/biochemistry/topics/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [LDH isoenzyme blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003499.htm) (MedlinePlus, US National Library of Medicine, 2026); [Creatine phosphokinase test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003503.htm) (MedlinePlus, US National Library of Medicine, 2026); [Alkaline Phosphatase: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/alkaline-phosphatase/) (MedlinePlus, US National Library of Medicine, 2026); [Troponin Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/troponin-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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