Veterinary Clinical Biochemistry and Laboratory Diagnosis-II

PLASMA ENZYMES -

Plasma Enzymes in Diagnosis

• Most enzymes are present in side the cells at much higher concentrations than in plasma.
• Some enzymes occur predominantly in cells of certain tissues, where they may be located in different compartments, such as cytoplasm or the mitochondria.
• Normal plasma enzyme levels reflect the balance between the rate of synthesis and release into plasma during cell turnover and the rate of clearance from the circulation.
• Plasma enzyme level depend on: 
  • Increased cell turnover.
  • Cellular proliferation.
  • Increased enzyme synthesis.
  • Obstruction to secretion.
  • Decreased clearance.
• Enzyme assays usually depend on the measurement of the catalytic activity of the enzyme, rather than the concentration of the enzyme.

Disadvantage of Enzyme Assays

• A major disadvantage in the use of enzymes for the diagnosis of tissue damage is their lack of specificity to a particular tissue or cell type.
• As several enzymes are common to many tissues, an increase in the plasma activity of a particular enzyme could reflect damage to any one of these tissues.
• This problem may be overcome to some extent by two ways: 
  • Different tissues may contain two or more enzymes in different proportions.
    • Thus alanine and aspartate transaminase are both present in cardiac muscle and hepatocytes, but there is relatively more alaninetransaminase in liver.
  • Some enzymes exists in different forms, known as isoenzymes.
    • Individual isoforms are characteristic of a particular tissue.
    • Though they may have similar catalytic activity, they will be differentiated by their physical and chemical properties like heat stability or sensitivity to inhibitors.
    • For example creatine kinase may be derived from skeletal muscle but one of its isoenzymes is found predominantly in the myocardium.
    • The distribution of enzymes with the cells may differ.
    • Alanine transaminase and lactate dehydrogenase are present only in cytoplasm.
    • Glutamate dehydrogenase is found only in mitochondria.