Purification and Initial Kinetic Characterization of Different Forms of O-Acetylserine Sulfhydrylase from Seedlings of Two Species of Phaseolus

Abstract

Purification of O-acetylserine sulfhydrylase (OASS) from seedlings of 2 spp. of Phaseolus [P. vulgaris and P. polyanthus] reveals the presence of 2 forms of this enzyme. The isolation and purification procedure gives purification of 7-160-fold for individual isoenzymes with specific activities ranging from 33 IU mg-1-775 IU mg-1 protein. Detailed study of the basic kinetic parameters of the OASS isoenzymes indicates that both forms from P. vulgaris (which are of about equal specific activity) display substrate inhibition by S2- above 1 mM and positive cooperativity at lower concentrations of S2-. With respect to O-acetylserine (OAS), the second substrate of the reaction, 1 P. vulgaris isoenzyme shows substrate inhibition by OAS concentrations above 10 mM, while the 2nd is unaffected by OAS concentrations up to 50 mM. The isoenzymes from P. polyanthus (one of which has a specific activity 24 times higher than the other) are slightly and approximately equally inhibited by both S2- and OAS.