The Fluidity of Serum Glutamic Oxaloacetic Transaminase.
Gender Norms & Racial Bias in the Study of the Modern "Serum Glutamic Oxaloacetic Transaminase"
Aspartate transaminase or aspartate aminotransferase, also known as AspAT/ASAT/AAT or glutamic oxaloacetic transaminase, is a pyridoxal phosphate -dependent transaminase enzyme that was first described by Arthur Karmen and colleagues in 1954. AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver, heart, skeletal muscle, kidneys, brain, red blood cells and gall bladder. Serum AST level, serum ALT level, and their ratio are commonly measured clinically as biomarkers for liver health. The tests are part of blood panels.
The half-life of total AST in the circulation approximates 17 hours and, on average, 87 hours for mitochondrial AST. Aminotransferase is cleared by sinusoidal cells in the liver. The amino group transfer catalyzed by this enzyme is crucial in both amino acid degradation and biosynthesis. In amino acid degradation, following the conversion of α-ketoglutarate to glutamate, glutamate subsequently undergoes oxidative deamination to form ammonium ions, which are excreted as urea. In the reverse reaction, aspartate may be synthesized from oxaloacetate, which is a key intermediate in the citric acid cycle.
Isoenzymes
Two isoenzymes are present in a wide variety of eukaryotes. In humans:
GOT1/cAST, the cytosolic isoenzyme derives mainly from red blood cells and heart.
GOT2/mAST, the mitochondrial isoenzyme is present predominantly in liver.
These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms, including E. coli, H. mediterranei, and T. thermophilus. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase.
Structure
X-ray crystallography studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria, pig heart cytosol, and E. coli. Overall, the three-dimensional polypeptide structure for all species is quite similar. AST is dimeric, consisting of two identical subunits, each with approximately 400 amino acid residues and a molecular weight of approximately 45 kD.
The two independent active sites are positioned near the interface between the two domains. Within each active site, a couple arginine residues are responsible for the enzyme's specificity for dicarboxylic acid substrates: Arg386 interacts with the substrate's proximal carboxylate group, while Arg292 complexes with the distal carboxylate. In either case, the transaminase reaction consists of two similar half-reactions that constitute what is referred to as a ping-pong mechanism. In the first half-reaction, amino acid 1 reacts with the enzyme-PLP complex to generate ketoacid 1 and the modified enzyme-PMP. In the second half-reaction, ketoacid 2 reacts with enzyme-PMP to produce amino acid 2, regenerating the original enzyme-PLP in the process. Formation of a racemic product is very rare.
The specific steps for the half-reaction of Enzyme-PLP + aspartate ⇌ Enzyme-PMP + oxaloacetate are as follows ; the other half-reaction proceeds in the reverse manner, with α-ketoglutarate as the substrate. However, it has been shown that the substrate binding step drives the catalytic reaction forward.
Clinical significance
AST is similar to alanine transaminase in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart, skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction, acute pancreatitis, acute hemolytic anemia, severe burns, acute renal disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins.
Laboratory tests should always be interpreted using the reference range from the laboratory that performed the test. Example reference ranges are shown below:
See also
Alanine transaminase
Transaminases
References
Further reading
External links
- Lab Tests Online
Bibliography:
Wikipedia
@baygross