Enzyme L1

Enzymes

• Enzymology is the study of enzymes in body fluids, commonly used for diagnosis and monitoring of conditions
• Measuring plasma enzyme activity has limits, as it lacks specificity
• Isoenzymes, differing molecular variants of enzymes, and measuring multiple enzymes improve diagnostic accuracy
• Functional plasma enzymes are present at all times in circulation, performing physiological functions in the blood
• Non-functional plasma enzymes lack specific function in plasma, and function inside cells
• Non-functional enzymes are released during normal cell turnover
• Low levels are present in serum, but levels rise in tissue injury, helping diagnose the site and extent of injury

Steady-state of Enzyme Activity

• Normally, plasma enzyme activity indicates a steady-state
• The rate of enzyme entry into plasma equals the rate of enzyme removal

Enzyme Activity Abnormalities

• Higher than normal plasma enzyme activity can be due to:
  • Proliferation of cells, increasing cell turnover or damage
  • Increased rate of cell turnover or damage
  • Increased enzyme synthesis (induction)
  • Reduced clearance from the plasma
• Lower than normal plasma enzyme activity can be due to:
  • Reduced enzyme synthesis, congenital deficiency, or inherited variants with low biological activity
  • Presence of macroenzymes, high-molecular-weight forms of native enzymes, often complexed with immunoglobulins, common in autoimmune diseases

Clinically Important Plasma Enzymes

Aminotransferases

• Transfer amino groups from one molecule to another
• Two important aminotransferases are aspartate aminotransferase (AST) and alanine aminotransferase (ALT)

Aspartate Aminotransferase (AST)

• Also known as glutamate oxaloacetate aminotransferase (GOT)
• Catalyzes the transfer of an amino group from aspartate to α-ketoglutarate, forming oxaloacetate and glutamate
• Found in high concentrations in:
  • Cardiac muscle
  • Erythrocytes
  • Skeletal muscle
  • Liver
  • Kidneys

Causes of Raised Plasma AST Activity

• Artefactual: In vitro release from RBC due to hemolysis
• Marked increase (greater than 5-10 times the upper reference limit or URL): Note: URL is the reference range for AST activity.
  • Circulatory failure with shock and hypoxia
  • Myocardial infarction
  • Acute viral or toxic hepatitis
• Moderate to slight increase (less than 5 times URL):
  • Cirrhosis
  • Liver tumors
  • Skeletal muscle disease

Alanine Aminotransferase (ALT)

• Also known as glutamate pyruvate aminotransferase (GPT)
• Catalyzes the transfer of an amino group from alanine to α-ketoglutarate, forming glutamate and pyruvate
• Found in high concentrations in:
  • Liver
  • Skeletal muscle
  • Kidneys
  • Cardiac muscle

Causes of Raised Plasma ALT Activity

• Marked increase (greater than 5-10 times URL): Note: URL is the reference range for ALT activity.
  • Circulatory failure with shock and hypoxia
  • Acute viral or toxic hepatitis
• Moderate to slight increase (less than 5 times URL):
  • Liver cirrhosis
  • Liver tumors

Lactate Dehydrogenase (LDH)

• Catalyzes the reversible conversion of lactate to pyruvate using NAD+ as a cofactor
• Found in many tissues, including:
  • Skeletal muscle
  • Kidneys
  • Brain
  • Cardiac muscle
  • Liver
  • RBCs

Causes of Raised Plasma Total LDH Activity

• Artefactual: In vitro hemolysis
• Marked increase (greater than 5-10 times URL): Note: URL is the reference range for LDH activity.
  • Circulatory failure with shock and hypoxia
  • Myocardial infarction
  • Some hematological disorders: Megaloblastic anemia, acute leukemia, and lymphomas
• Moderate to slight increase (less than 5 times URL):
  • Viral hepatitis
  • Malignancy of any tissue

Isoenzymes of LDH

• Five main isoenzymes can be detected by electrophoresis: LDH1 to LDH5
• Each isoenzyme consists of two different kinds of polypeptide chains, designated as H and M
• Isoenzyme and tissue distribution:
  • LDH1: H4 - Heart - Elevated in Myocardial Infarction (MI)
  • LDH2: H3M1 - Heart, Kidney, RBCs - Elevated in MI and hemolytic anemia
  • LDH3: H2M2 - Widely distributed - Elevated in malignancy of many tissues
  • LDH4: H1M3 - Widely distributed - Little diagnostic significance
  • LDH5: M4 - Liver, Skeletal muscle - Elevated in skeletal muscle and liver diseases

Creatine Kinase (CK)

• Catalyzes the reversible phosphorylation of creatine to creatine phosphate, using ATP
• Found in high concentrations in:
  • Skeletal muscle
  • Cardiac muscle
  • Brain

Isoenzymes of Creatine Kinase

• Three principal isoenzymes: BB, MB, and MM
• Each isoenzyme consists of two polypeptide chains: B (brain) or M (muscle)
• Isoenzyme and tissue distribution:
  • BB (CK-1): Predominant in brain
  • MB (CK-2): Predominant in cardiac muscle
  • MM (CK-3): Predominant in skeletal muscle

Other Forms of CK

• Mitochondrial CK: Found in hepatic disease, certain tumors, and critically ill patients
  • Type 1 Macroenzyme: Associated with autoimmune disease, like rheumatoid arthritis, thought to be CK complexed with IgG
  • Type 2 Macroenzyme: An oligomer of mitochondrial CK

Causes of Raised Plasma Creatine Kinase Activities

• Marked increase (greater than 5-10 times URL): Note: URL is the reference range for CK activity.
  • Dermatomyositis and polymyositis
  • Shock and circulatory failure
  • Myocardial infarction
• Moderate to slight increase (less than 5 times URL):
  • Muscle injury
  • Physical exertion
  • After an intramuscular injection
  • Hypothyroidism
  • Certain drugs: statins, ciclosporin, cocaine

Amylase

• Breaks down starch and glycogen
• Serum amylase originates mainly from the pancreas (P-isoamylase) and salivary glands (S-isoamylase)
• Can also be extracted from other tissues: gonads, Fallopian tubes, skeletal muscle, and adipose tissue
• Excreted in urine due to its low molecular weight

Causes of Raised Plasma Amylase Activity

• Marked increase (greater than 5-10 times URL): Note: URL is the reference range for amylase activity.
  • Acute pancreatitis
  • Severe glomerular impairment
  • Diabetic ketoacidosis
  • Perforated peptic ulcer
• Moderate to slight increase (less than 5 times URL):
  • Other acute abdominal disorders: acute cholecystitis, intestinal obstruction
  • Salivary gland disorders: mumps, salivary calculi
• Decreased amylase activity:
  • Chronic pancreatic disease

Macroamylasaemia

• High plasma amylase activity due to low renal excretion of a macroenzyme form, despite normal glomerular function
• Symptomless, thought to be the enzyme bound to IgA, forming a complex with a molecular weight of about 270 kDa

Lipase

• Derived from the pancreas, more specific for pancreatic pathology
• Longer half-life than amylase, making it more useful for diagnosis of late-presenting acute pancreatitis