Mechanism and biological role
The transamination process is an exchange reaction that occurs without the direct consumption of ATP. The main donors of amino groups in cells are glutamate, aspartate, and alanine. This process is most active in the liver, although it occurs in all tissues of the body.
In metabolism, transamination fulfills two opposing tasks:
- Anabolic (synthesis): formation of non-essential amino acids during their local deficiency. The amino group is simply transferred to a "precursor" (the corresponding $\alpha$-keto acid), ensuring the redistribution of amine nitrogen.
- Catabolic (breakdown): transamination serves as the first step in the degradation of amino acids. The removal of the amino group leaves a carbon skeleton in the form of an $\alpha$-keto acid, which is subsequently oxidized in the tricarboxylic acid cycle to generate energy, or used for the synthesis of glucose (gluconeogenesis) and ketone bodies.
Aminotransferase enzymes (ALT and AST)
The reactions are catalyzed by specific intracellular enzymes — aminotransferases (transaminases). There are over 10 varieties of them, and all require the presence of a coenzyme — pyridoxal phosphate (a derivative of vitamin $B_6$).
Two enzymes are of the greatest clinical significance:
- Alanine aminotransferase (ALT): catalyzes the transfer of an amino group between alanine and $\alpha$-ketoglutarate, yielding pyruvate and glutamate.
- Aspartate aminotransferase (AST): facilitates the reaction between aspartate and $\alpha$-ketoglutarate, resulting in the synthesis of oxaloacetate and glutamate.
AST exists in two forms: cytosolic (c-AST) and mitochondrial (m-AST). In hepatocytes and myocardial cells, the mitochondrial fraction accounts for up to 80% of the enzyme.
Principles of enzymodiagnostics
Normally, aminotransferases function inside cells, so their activity in blood serum is minimal (5–40 IU/L). If the cell membrane is damaged (cytolysis syndrome), the enzymes are released into the bloodstream in large quantities. Since transaminases are organ-specific, their levels can be used to assess the localization of pathology.
- Liver pathologies (viral hepatitis): a sharp increase in ALT (6–8 fold) is a key marker. The peak is observed on days 2–3, and normalization is slow (more than 15 days).
- Myocardial infarction: characterized by a pronounced increase in AST (8–10 fold) against the background of a slight increase in ALT. Important: transaminase activity does not increase in angina pectoris or heart defects.
De Ritis ratio
For differential diagnosis, physicians use the De Ritis ratio — the ratio of AST to ALT activities in blood serum.
| Condition | Ratio | Explanation |
|---|---|---|
| Normal | $1,33 \pm 0,42$ | Physiological ratio of enzymes |
| Acute hepatitis | Decreases ($\approx 0.6$) | Dominant release of ALT due to liver damage |
| Liver cirrhosis | Approaches $1.0$ | Profound necrosis leads to the release of the heavy mitochondrial fraction of AST |
| Myocardial infarction | Increases (up to $6.0$) | Massive release of heart-specific AST |
As additional markers of liver pathology, $\gamma$-glutamyl transferase ($\gamma$-GT) is often used, which reflects cholestasis and toxic damage, while specific isoforms of creatine kinase (CK-MB) and lactate dehydrogenase (LDH-1) are used for the myocardium.
Briefly: answers to topical questions
What is the molecular mechanism of amino group transfer by pyridoxal phosphate?
The molecular mechanism consists of the reversible chemical conversion of the coenzyme's aldehyde group into an amino group.
The original pyridoxal phosphate has a pyridine ring in its structure containing an aldehyde group ($-CHO$) at position 4. During transamination, this group accepts nitrogen from the donor amino acid. As a result of the reaction, pyridoxamine phosphate is formed, containing an aminomethyl group ($-CH_2-NH_2$) at the same fourth position.
To remember the exceptions (amino acids that do not undergo transamination), use the mnemonic: Lysine, Threonine, Proline.
Class and exam questions
Why does the level of AST rise rather than ALT during a myocardial infarction?
AST is an organ-specific enzyme for the heart—its concentration in cardiomyocytes is significantly higher. In myocardial necrosis, this enzyme enters the systemic circulation in large quantities.
What does the appearance of mitochondrial AST (m-AST) in the blood indicate?
The release of m-AST indicates severe cell damage (necrosis), in which not only cell membranes but also intracellular organelles are destroyed. This is characteristic of severe conditions such as liver cirrhosis.
Is ATP energy required for transamination reactions?
No, aminotransferases catalyze fully reversible reactions that do not use ATP as a direct energy source.
What else to study on this topic
- Oxidative deamination of amino acids
- Glutamate as a key molecule in nitrogen metabolism
- The link between amino acid metabolism, the Krebs cycle (TCA cycle), and gluconeogenesis
- Diagnostic significance of LDH isoenzymes (LDH-1, LDH-4, LDH-5)
- Meister cycle and amino acid transport involving $\gamma$-GT
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