Studies on phosphoserine aminotransferase of sheep brain.

Phosphoserine aminotransferase has been purified about 500-fold from sheep brain extracts by ammonium sulfate precipitation and Sephadex and diethylaminoethyl cellulose chromatography. On ultracentrifugation, the enzyme migrated in a single peak with a sedimentation constant of 4.2 S. The molecular weight determined by the Trautman modification of the Archibald method is 96,000. The spectrum of the aminotransferase shows an absorption peak at 415 mp. The enzyme could be inactivated by dialysis against cysteine, and the activity was restored upon incubation with pyridoxal phosphate. This and other evidence suggest that the latter is a coenzyme for this enzyme. The enzyme utilizes glutamic acid preferentially for transamination. The reaction rate with alanine as amino group donor is about 10% of that of glutamate. The optimum activity is at pH 8.15. In characterizing the enzyme, the transamination reaction was followed in both directions and the stoichiometry, the equilibrium constant, and the Michaelis constants for 3-phosphohydroxypyruvate (0.25 mu) and glutamate (0.7 man) have been determined.