Aspartate Transaminase (AST/GOT/SGOT)

EC 2.6.1.1

Aspartate Transaminase (AST) has multiple names including Aspartate Aminotransferase, Glutamate-Oxaloacetate Transaminase (GOT), and Serum Glutamate-Oxaloacetate Transaminase (SGOT). As a member of the aminotransferase family, AST catalyzes the reversible transfer of the amino group from glutamate to oxaloacetate while replacing the amino group of glutamate with a carbonyl group:

The reaction catalyzed by AST requires the cofactor known as pyridoxal-5-phosphate (P5P), the active form of vitamin B6. It binds covalently, but reversibly to an active-site lysine. This helps catalyze the reaction by accepting the amino group from the amino acid then transferring it to the carbonyl compound¹. Any sample, whether serum or purified extract, to which P5P is added may show a marked increase in AST activity if some or all of the enzyme does not already have the cofactor bound to it.

Under normal conditions AST is found in most tissues and fluids of the body except urine. However, unlike ALT, it can be found at comparable significance in the heart, kidneys, pancreas and skeletal muscle². Consequently, elevated levels of serum AST can be caused by pathologies of many different organs. However, it is still considered to be predominantly a marker of liver function. One method of analysis involves measuring a ratio of AST:ALT^4,8 but the effectiveness of this is somewhat controversial^5,6. An additional feature of AST is that there is both a cytoplasmic isoenzyme as well as a mitochondrial isoenzyme. Although it is generally true that the cytoplasmic form of AST is the predominant one in serum, mitochondrial AST can be elevated and sometimes higher than the cytoplasmic form and this information can be diagnostically useful⁷. Although serum AST levels predictably rise after myocardial infarction, the elevation is far too slow to be useful for diagnosis³. Far more useful markers exist for diagnosing a heart attack.

Measuring ALT is usually done by an indirect enzymatic method. There are no useful spectrophotometric changes that occur during the transamination reaction. However, a very useful and versatile technique is to couple a product of a reaction to another enzymatic reaction that either reduces or oxidizes NAD+/NADH respectively. There are many such reactions. Reduction of NAD+ results in an increase in absorption at 340 nm. Oxidation of NADH has the opposite effect. In the case of AST, the product pyruvate can be very conveniently coupled to the malic dehydrogenase reaction. This results in the oxidation of NADH and the AST-MDH coupled reaction is followed by monitoring the decrease in absorbance at 340 nm.

REFERENCES

1. 1. Walsh, C.: Enzymatic Reaction Mechanisms, W.H. Freeman and Company, 1979.
2. 2. King, J.: Practical Clinical Enzymology, London, D. Van Nostrand Company, 1965.
3. 3. Burtis, C.A., Ashwood, E.R. and Bruns, E.R. Tietz Textbook of Clinical Chemistry, W.B. Saunders Company, 2006.
4. 4. Giannini, E., et. al. Progressive Liver Functional Impairment is Associated with an Increase in AST/ALT Ratio. Digestive Diseases and Sciences, 44, 1249-1253, 1999.
5. 5. Gordon, J.H.P., et. al. Aspartate Aminotransferase: Alanine Aminotransferase Ratio in Chronic Hepatitis C Infection: Is it a Useful Predictor of Cirrhosis? Journal of Gastroenterology and Hepatology, 15, 386-390, 2000.
6. 6. Yu, A.S. and Keeffe, E.B. Elevated AST or ALT to Nonalcoholic Fatty Liver Disease: Accurate Predictor of Disease Prevalence? American Journal of Gastroenterology, 98, 955-956, 2003.
7. 7. Panteghini, M., et. al. Determination of Aspartate Aminotransferase Isoenzymes in Hepatic Disease- Preliminary Findings. Clinica Chimica Acta, 128, 133-140, 1983.
8. 8. Giannini, E., et. al. Validity and Clinical Utility of the Aspartate Aminotransferase- Alanine Aminotransferase Ratio in Assessing Disease Severity and Prognosis in Patients with Hepatitis C Virus-Related Chronic Liver Disease. Archives of internal Medicine, 163, 218-224, 2003.