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Goldberg DM, Remtulla MA, Lustig V. The diagnostic accuracy of three recommended methods for serum aspartate aminotransferase assays in patients suspected of myocardial infarction and hepatobiliary diseases.Clin Biochem. 1988 Oct;21(5):323-8

Phillips RS. Chemistry and Diversity of Pyridoxal-5'-phosphate Dependent Enzymes. Biochim Biophys Acta. 2015 Jan 20. pii: S1570-9639(15)00011-4. 

p5p is a co factor or vitamin B6 which is present in all normal  human serum  But it varies in individual or in the patient group in an considerable amount.Here,With p5p means,the addition of p5p involves the full catalytic enzymatic reaction and also stabilizes the variation of enzymes at the time of measurement.even it can used to measure low values also.

when it comes to without p5p modified methods ,it was optimized for performance and stability,.even though the reagents like two reagent type or three reagent type methods,some of the studies shows that it understimate the values in the patient group,especially those who are deficient with p5p in their serum.in without p5p ,the rate of NADH oxidation  is directly proportional to the catalytic enzyme activity

The rate of the decrease of NADH is measured photometrically and is direct proportional to the ASAT-activity in the specimen.Addition of pyridoxal-5-phosphate (P-

5-P) stabilizes the transaminases and avoids falsely low values in samples

containing insufficient endogenous P-5-P, e.g. from patients with myocardial infarction, liver disease and intensive care patients