Isoenzymes in Clinical Diagnosis

Introduction to Enzymes

• Enzymes act as biocatalysts, increasing reaction rates without changing themselves.
• Primarily proteins, enzymes accelerate specific chemical reactions by lowering activation energy.
• Found in all living cells, they convert substrates into products without being consumed.
• Enzymes are globular proteins, highly selective, and can be denatured, losing functionality.

Enzyme Distribution

• Enzymes categorized based on their site of action:
  • Intracellular enzymes: Function within cells (e.g., metabolic enzymes).
  • Extracellular enzymes: Produced in cells but act outside (e.g., digestive enzymes).

Nomenclature and Classification

• Enzymes typically end in -ase derived from the substrate they act on (e.g., lipase for lipids).
• Historical names like pepsin and trypsin provide limited functional insight.
• Six major enzyme classes identified by the International Union of Biochemistry (IUB):
  • Oxidoreductases: Catalyze oxidation-reduction reactions.
  • Transferases: Transfer functional groups other than hydrogen.
  • Hydrolases: Split bonds by adding water.
  • Lyases: Add or remove groups without hydrolysis.
  • Isomerases: Catalyze interconversion of isomers.
  • Ligases: Link two molecules using energy from ATP.

Enzyme Structure and Activity

• Enzymes can be simple proteins or conjugated proteins (holoenzymes).
• Holoenzymes consist of the protein part (apoenzyme) and a non-protein cofactor.
• Cofactors can be organic (coenzymes) or inorganic.

Important Coenzymes

• Hydrogen carriers:
  • NAD, NADP, FAD, FMN, lipoic acid, coenzyme-Q.
• Group carriers (mostly derived from vitamins):
  • Coenzyme A (acyl group), Thiamine pyrophosphate (TPP: CO2), Biotin (CO2), Pyridoxal phosphate (PLP: -NH2), Folic acid (one carbon), Cobalamine (methyl group).

Enzyme Inhibition

• Competitive Inhibition: Sulfanilamide blocks enzyme using PABA to synthesize folic acid.
• Non-competitive Inhibition:
  • Inhibitor binds away from the active site, changing enzyme shape; Km remains unchanged, Vmax decreases.
  • Heavy metals (Ag+, Pb2+, Hg2+) can act as non-competitive inhibitors.
• Irreversible Inhibition: Inhibitors bind covalently, permanently inactivating enzymes (e.g., iodoacetate, DIFP).

Isoenzymes

• Isoenzymes vary in structure and properties but catalyze the same reaction.
• Examples include lactate dehydrogenase (LDH) and creatine kinase (CK).

Enzymes in Clinical Diagnosis

• Multiple serum enzymes provide better disease diagnosis than a single enzyme measurement.

Enzymes in Myocardial Infarction (MI)

• Key markers include:
  • Creatine phosphokinase (CPK): First enzyme elevated within 6-18 hours, peaks in 24-30 hours.
  • Aspartate transaminase (AST): Peaks within 48 hours, normalization takes 4-5 days.
  • Lactate dehydrogenase (LDH): Peaks around 3-4 days, normalizes after 10-15 days.

Enzymes in Liver Diseases

• Elevated serum enzymes diagnose liver dysfunction:
  • Alanine transaminase (ALT), Aspartate transaminase (AST), Lactate dehydrogenase (LDH).
• Specific markers for cholestasis: Alkaline phosphatase, 5’-Nucleotidase.
• Serum γ-glutamyl transpeptidase indicates alcoholic liver disease.

Enzymes in Muscle Diseases

• Elevated muscle enzymes in muscular dystrophies include:
  • Creatine phosphokinase, Aldolase, Aspartate transaminase.

Enzymes in Cancers

• Acid phosphatase: Marker for prostatic carcinoma.
• Neuron-specific enolase: Marker for lung cancer, neuroblastoma.

Enzymes in Bone and Pancreatic Diseases

• Alkaline phosphatase (ALP): Increased in rickets disease.
• Amylase and Lipase: Significantly elevated in acute pancreatitis.

Understanding the physical and chemical properties of isoenzymes, including lactate dehydrogenase and creatine kinase, for accurate disease diagnosis.