Transaminases (ALT, AST) as indicators of liver and myocardium diseases
WHAT ARE TRANSAMINASES
Transaminases are intracellular enzymes that perform catalytic transamination which is a transfer of an amino group (NH2) from a molecule of amino acid to a molecule of α-keto acid without transitional formation of ammonia. This is how interconnection between nitrogen metabolism and carbohydrate metabolism is carried through.
The term "transaminases" is considered obsolete. The new name is "aminotransferases". Yet the old term still keeps its place in medical practice.
Transaminases are present in every cell of every life-form, ranging from bacteria to complex multi-celled organisms.
Each set of transaminases specializes on transamination of just one amino acid or a group of amino acids that share similar structure. Aminotransferases are named in accordance with amino acid that participates a reaction of transamination. Thus, alanine aminotransferase (ALT, ALAT) is intended for alanine amino transferase, aspartate aminotransferase (AST, ASAT) is for aspartic acid, glatamate aminotransfetase is for glutamic acid and so on.α-Ketoglutaric acid, pyruvic acid or some other acid can act as a recipient for amino group .
Reaction of transamination occurs with participation of coenzyme — vitamin B6 (Pyridoxine). Vitamin B6 in its active form of Pyridoxal phosphate makes up with aminotransferase a single "enzyme-coenzyme" set.
CONDITIONS ACCOMPANIED WITH ELEVATED TRANSAMINASES
Aminotransferases are tissue-specific. In the human's body alanine aminotransferase shows its maximum activity in the liver, as apartate aminotransferase does in heart muscle (myocardium). Breakdown of the cells of these organs in some conditions underlies a presence of large quantities of intracellular enzymes in blood.
Activity of ALT and AST in blood rises:
- In liver diseases, especially in viral hepatitis. Elevated transaminases in blood is among the very first laboratory signs of hepatitis. This symptom manifests itself long before bilirubin elevation and jaundice:
- acute viral hepatitis
- chronic hepatitis
- toxic hepatitis
- drug-induced hepatitis
- bacterial hepatitis
- intrahepatic cholestasis
- prolonged obstructive jaundice
- liver tumors
- In myocardial infarction. 2-20x increase of transaminases in blood ocuurs on 2-3 day in myocardial infarction. In angina, in contrast to myocardial infarction, transaminase level remains normal.
Temporary elevation of transaminases is also a characteristic of burn disease, massive trauma of skeletal muscles, myoglobinuria, myositis, myopathy, muscular dystrophy, bone tumors, conditions accompanied with hemolysis (erythrocytes breakdown), pancreatitis, after abdominal surgery.
DE RITIS RATIO (DRr)
An important contribution to understanding the biochemistry of transaminases and their role was made by Italian scientist Fernando De Ritis.
Researches of De Ritis have shown that it's not only amounts of AST and ALT what has a dianostic value, but also their ratio. De Ritis has established a diagnostic criterion named after him De Ritis ratio.
De Ritis ratio represents a proportion between AST and ALT amounts.
De Ritis ratio should be calculated only when the measured values of transaminases are higher than normal.
Numerous biochemical reserches of the have proven with high reliability that De Ritis ratio:
- DRr<1 is a case for the viral hepatitis;
- DRr≥1 — for the chronic hepatitis and dystrophyc diseases of liver;
- DRr≥2 coupled with a low blood albumin (<35 g/l) — for alcoholic damage of liver.
Besides liver conditions, de Ritis ratio is also useful in diagnostics of heart disease. In myocardial infarction its value stands at DRr>1,3. Since in cardiac muscle AST prevails over ALT (and vice versa in liver), it causes a massive release of, first of all, aspartate aminotransferase, from demaged tissues into a bloodstream. Of course, in this case De Ritis ratio matters if there is no chronic liver condition.
In either case, De Ritis ratio should be interpreted together with other laboratory data.