Pathophysiological basis of the method
In a healthy body, blood serum contains only those enzymes that directly perform their physiological functions in it (for example, blood clotting factors). The vast majority of other catalysts are cellular enzymes. They work strictly inside cells and normally practically do not cross the intact cell membrane, which is why they are either absent in the bloodstream or detected 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 violation of the barrier function of membranes. This phenomenon is called cytolysis syndrome. As a result, the intracellular contents pour into the intercellular space and then enter the blood or other biological fluids (for example, urine). Laboratory tests record a sharp increase in the amount and catalytic activity of specific tissue enzymes.
Criteria for the diagnostic value of enzymes
For an enzyme to be reliably used in clinical practice to make a diagnosis, it must meet a number of 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 in direct proportion to 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, the enzyme activity must remain at a stable level for a sufficiently long time (during the day) and reliably differ from normal values.
- Diagnostic significance of intracellular localization. The depth of tissue damage is assessed by where the enzyme came from. If exclusively cytosolic enzymes appear in the plasma, this indicates an increase in plasmalemma permeability, which is characteristic of inflammation. If mitochondrial or nuclear markers are detected, this indicates deep damage to cellular structures — necrosis.
Markers of internal organ damage
Clinical correspondence between an organ and a specific enzyme makes it possible to accurately localize the focus of pathology. The main enzymes used to assess the state of various systems:
- 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.
Briefly: answers to topical questions
Which lactate dehydrogenase (LDH) isoenzymes are specific for myocardial infarction?
The LDH-1 isoenzyme is used to diagnose myocardial infarction.
- LDH-1 is the cardiac isoform of lactate dehydrogenase; the $H_4$ form predominates in the myocardium.
- This isoform has a high affinity for lactate.
The appearance of tissue-specific LDH isoforms in the blood indicates damage to the corresponding organ; LDH-1 is indicated as a marker of myocardial infarction.
Which creatine kinase (CK) isoenzyme has the highest specificity for heart muscle?
The CK-MB isoenzyme has the highest specificity for heart muscle.
- CK-MB is an isoform of creatine kinase localized in the heart muscle (myocardium).
- The creatine kinase molecule is a dimer; the MB isoform is formed by M and B subunits.
In myocardial infarction, the level of the MB isoform in blood plasma increases; it is an important diagnostic marker of myocardial damage.
Class and exam questions
What is the difference between the appearance of cytosolic and mitochondrial enzymes in the blood?
Cytosolic enzymes are released during mild membrane damage, which indicates an inflammatory process. The appearance of mitochondrial or nuclear enzymes indicates deep cell damage, up to complete tissue necrosis.
Which markers indicate the development of myocardial infarction?
Enzyme markers of infarction are creatine kinase (CK), lactate dehydrogenase (LDH), as well as AST and ALT. At the same time, the very first indicator of damage is the protein troponin, which is not an enzyme.
Why are intracellular enzymes normally not detected in plasma?
In a healthy state, cell membranes are not damaged, so enzymes remain inside cells, performing their functions. They penetrate into the bloodstream only in minimal quantities, insufficient for diagnostic significance.
What else to study on this topic
- Structure and functions of lactate dehydrogenase (LDH) isoenzymes
- Isoenzyme spectrum of creatine kinase in muscle and heart tissue
- Mechanisms of cytolysis syndrome at the molecular level
- Diagnostics of liver diseases: clinical significance of AST and ALT
- Role of pancreatic amylase and lipase in norm and in pancreatitis
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