beta-Cystathionase In Vivo Inactivation by Rhizobitoxine and Role of the Enzyme in Methionine Biosynthesis in Corn Seedlings

Abstract

Rhizobitoxine has previously been shown to inactivate irreversibly beta-cystathionase isolated from spinach. In the present studies, rhizobitoxine was shown to inhibit partially beta-cystathionase of spinach and corn seedlings in vivo. An activity of 30 to 40% of normal remained in toxin-treated seedlings of both spinach and corn. Possible reasons for the partial inhibition are discussed.Rhizobitoxine-treated and control corn seedlings were allowed to assimilate (35)SO(4) (2-) for 3 or 6 hours, and the radioactivity incorporated into sulfur amino acids at these times was studied. The most striking effect of rhizobitoxine was an increase (up to 22-fold) in radioactive cystathionine. Accumulation of radioactivity in methionine was only slightly inhibited by rhizobitoxine treatment. The results strongly suggest that the transsulfuration pathway contributes to methionine biosynthesis, and that metabolism via this pathway is impaired, but not entirely eliminated, by rhizobitoxine treatment.The present data do not permit decisions about the relative contributions of the transsulfuration and the direct sulfhydration pathways to methionine biosynthesis, or whether the pathological effects of rhizobitoxine are due chiefly to inhibition of beta-cystathionase.