Plasma enzymes

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.
Last modified: Thursday, 1 December 2011, 12:10 PM