Methiomine Synthesis in Lemna1 INHIBITION OF CYSTATHIONINE y-SYNTHASE BY PROPARGYLGLYCINE

Propargylglycine, vinylglycine, and cysteine each cause irreversible in-activations of cystathionine -synthase (and, in parallel, of O-phosphoho-moserine sulfhydrylase) activities in crude extracts of Lemna paucicostwa. Inactivation by propargylglycine or vinylglycine is completely prevented by 40 millimolar 0-phospho- or O-succinyl-L-homoserine; that by cysteine is only partially prevented. Propargylglycine (PAG), the most potent ofthese inhibitors, causes rapid and drastic inactivation of both activities in intact Lemna. Studies of plants growing in steady states in the presence of various concentrations (0-150 nanomolar) of PAG showed that 16% of control activity is necessary and sufficient to maintain normal rates of growth and methionine biosynthesis, and that 10% of control activity is essential for viability. Addition of either 2 micromolar methionine or 31 micromolar cystine to growth medium containing 150 nanomolar PAG permits growth at 75 to 100% of control rates when enzyme activity is less than 10% of control. Whereas methionine presumably rescues by directly providing the missing metabolite, cystine may rescue by enhancing substrate accumulation and thereby promoting flux through residual cystathionine ysynthase. The results indicate that the down-regulation of cystathionine rysynthase to 15% of control which occurs when plants are grown in 2 micromolar methionine (Thompson, Datko,