Isoenzymes & Clinical

Isoenzymes & Clinical Enzymology

Isoenzymes (isozymes) are multiple molecular forms of an enzyme that catalyze the same reaction but differ in amino acid sequence, physical properties (electrophoretic mobility, heat stability, Km), and tissue distribution. They arise from different genes or different post-translational processing.

Lactate Dehydrogenase (LDH)

LDH catalyzes: Pyruvate + NADH ⇌ Lactate + NAD+. Tetramer of two subunits: H (heart) and M (muscle). 5 isozymes:

  • LDH1 (H4): Heart, RBCs — favors pyruvate oxidation
  • LDH2 (H3M1): Heart, RBCs
  • LDH3 (H2M2): Brain, kidney
  • LDH4 (HM3): Skeletal muscle, liver
  • LDH5 (M4): Liver, skeletal muscle — favors lactate formation

In Myocardial Infarction: LDH1 > LDH2 ("flipped pattern") after 24–48 h. (Now replaced by Troponin in clinical practice.)

Creatine Kinase (CK)

Dimer of B (brain) and M (muscle) subunits. 3 isozymes:

  • CK-BB (CK1): Brain — elevated in brain injury
  • CK-MB (CK2): Heart specifically — rises within 4–6h, peaks 12–24h in MI; used for diagnosis of MI
  • CK-MM (CK3): Skeletal muscle — elevated in muscular dystrophy, rhabdomyolysis

Alkaline Phosphatase (ALP)

Hydrolyzes phosphate esters at alkaline pH. Isoforms from: Liver (bile duct disease), Bone (osteoblasts — high in Paget's disease, growing children), Intestine, Placenta. Elevated in: Cholestasis, bone disease, pregnancy.

Aminotransferases (Transaminases)

  • ALT (Alanine aminotransferase, SGPT): More liver-specific. Best marker of hepatocellular damage (hepatitis). Cofactor: PLP (B6).
  • AST (Aspartate aminotransferase, SGOT): Found in liver, heart, muscle, kidney. Less specific. High in alcoholic hepatitis; AST:ALT ratio > 2 is characteristic.

Other Clinically Important Enzymes

  • Amylase & Lipase: Elevated in pancreatitis. Lipase more specific for pancreas.
  • Gamma-GT (GGT): Sensitive marker of liver disease; elevated in alcohol abuse.
  • Acid phosphatase (ACP): Lysosomes, prostate. PSA (prostate-specific antigen) used for prostate cancer screening.
  • Troponin I and T: Most sensitive and specific cardiac biomarkers for MI; gold standard.

Enzyme Diagnostics — Principles

  • Plasma enzymes normally present at low levels (from cell turnover)
  • Greatly elevated when organ damage releases intracellular enzymes
  • Diagnosis uses pattern of enzyme changes (rise and fall time course)