Pathophysiological Basis of the Method
In a healthy body, blood serum contains only those enzymes that directly perform their physiological functions within it (for example, blood clotting factors). The vast majority of other catalysts are cellular enzymes. They function strictly inside cells and normally virtually never cross the intact cell membrane; therefore, they are either absent in the bloodstream or detected only in trace (minimal) amounts.
The situation changes radically when pathology develops. Any damage to cellular structures—whether an inflammatory process or tissue death (necrosis)—is accompanied by a disruption of barrier membrane function. This phenomenon is termed cytolysis syndrome. As a result, the intracellular contents spill into the intercellular space and subsequently enter the blood or other biological fluids (such as urine). Laboratory tests record a sharp increase in the quantity and catalytic activity of specific tissue enzymes.
Criteria for Diagnostic Value of Enzymes
For an enzyme to be reliably used in clinical practice for diagnosis, it must meet several strict criteria:
- Organ specificity. An ideal diagnostic marker should have predominant or absolute localization within one specific organ. The isoenzyme spectrum—the predominance of certain isoenzymes in various tissue types—is also taken into account.
- Proportionality of release. The number of molecules released into the bloodstream must be directly dependent on the scale and degree of organ damage. In addition, their concentration must be sufficient for reliable detection.
- Stability in biological fluids. After entering the plasma, enzyme activity must remain stable for a sufficiently long time (over 24 hours) and differ significantly from normal values.
- Diagnostic significance of intracellular localization. The depth of tissue damage is assessed by the specific cellular origin of the released enzyme. If exclusively cytosolic enzymes appear in the plasma, it indicates increased plasmalemma permeability, which is characteristic of inflammation. If mitochondrial or nuclear markers are detected, it indicates profound damage to cellular structures, i.e., necrosis.
Markers of Internal Organ Damage
The clinical correlation between an organ and a specific enzyme allows precise localization of the pathological focus. The main enzymes used to assess the state of various systems include:
- Heart diseases (Myocardial Infarction):
- Lactate dehydrogenase (LDH)
- Creatine kinase (CK)
- Aspartate aminotransferase (AST)
- Alanine aminotransferase (ALT)
- Important note: One of the earliest markers of infarction is the protein troponin, but it does not belong to the class of enzymes.
- Liver diseases:
- Alanine aminotransferase (ALT)
- Aspartate aminotransferase (AST)
- Gamma-glutamyl transferase (GGT)
- Acetylcholinesterase
- Pancreatic diseases:
- Pancreatic amylase
- Lipase
- Prostate diseases:
- Acid phosphatase