Evidence for the Identity of O-Acetylserine Sulfhydrylase with O-Acetylhomoserine Sulfhydrylase in Yeast

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Abstract
O-Acetyl-L-homoserine (OAH) sulfhydrylase was highly purified from baker's yeast by a modification of our previous method. When assayed in phosphate buffer, but not in Tris-HCl buffer, the purified enzyme catalyzed the O-acetyl-L-serine (OAS) sulfhydrylase reaction at a rate which was about 15% that of the OAH sulfhydrylase reaction. The apparent absence of OAS sulfhydrylase activity in Tris-HCl buffer was found to be due to the instability of OAS in this buffer at alkaline pH's. Throughout the purification steps the OAH and OAS sulfhydrylase activities were purified in parallel, and neither DEAE-cellulose column chromatography of a partially purified preparation nor Sepharose 4B gel filtration of the final preparation could separate the two activities. Upon polyacrylamide gel electrophoresis the purified enzyme was separated into three protein bands, of which only the main band possessed the two activities. Moreover, the two activities behaved in the same way on heat treatment. Finally, the levels of OAS sulfhydrylase activity in extracts of various methionine auxotrophs of yeast varied in parallel with those of OAH sulfhydrylase activity. It is concluded that OAS and OAH sulfhydrylase activities of yeast are catalyzed by the same enzyme protein.

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