Effects of Buffers on Aspartate Aminotransferase Activity and Association of the Enzyme with Pyridoxal Phosphate

Using purified enzymes of human origin and patients' sera, we examined factors influencing the in vitro association of pyridoxal phosphate with aspartate aminotransferase (EC 2.6.1.1). The rate of association was markedly retarded by phosphate buffer in comparison with tris(hydroxymethyl)aminomethane or six other buffers. Pyridoxal phosphate at an incubation concentration of 130 mumol/liter reactivated the entire apoenzyme portion of an apoenzyme/holoenzyme mixture within 5 min in tris(hydroxymethyl)aminomethane; in contrast, less than 20% was associated during 15 min in phosphate. Activity measured in tris(hydroxymethyl)aminomethane-buffer without exogenous pyridoxal phosphate was 4% greater than that in phosphate and was slightly increased by increasing the pH of the assay mixture from 7.5 to 8.0. Aspartate in the incubation medium did not retard the stimulation in tris(hydroxymethyl)aminomethane buffer. While the magnitude of stimulation varied greatly among sera, a consistent mean stimulation of 30% for groups of sera with normal activities was found when asparate at 125 mmol/liter, 2-oxoglutarate at 6.7 mmol/liter and tris(hydroxymethyl)aminomethane at 90 mmol/liter were used, an increase over the 16% with phosphate buffer [Clin. Chem. 19, 92 (1973)]. Absorbance spectra suggest pyridoxal phosphate exists as the Schiff base of tris(hydroxymethyl)aminomethane or aspartate, or both, under conditions of assay incubation (without addition of 2-oxoglutarate). Nonenzymatic catalysis of the reaction by pyridoxal phosphate alone or a formation of a protein/pyridoxal phosphate adduct was discounted with use of a D-asparate substrates.