Methionine Biosynthesis in Lemna

Abstract

Regulation of enzymes of methionine biosynthesis was investigated by measuring the specific activities of O-phosphohomoserine-dependent cystathionine .gamma.-synthase [C], O-phosphohomoserine sulfhydrylase [P] and O-acetylserine sulfhydrylase [A] in L. paucicostata Hegelm. 6746 grown under various conditions. For C adding external methionine (2 .mu.M) decreased specific activity to 15% of control; blocking methionine synthesis with 0.05 .mu.M L-aminoethoxyvinylglycine or with 36 .mu.M lysine plus 4 .mu.M threonine (Datko et Mudd 1981) caused a 2- to 3-fold increase in specific activity; and blocking methionine synthesis and adding external methionine led to the decreased specific activity characteristic of methionine addition alone. Activity in extracts from control cultures was unaffected by addition of methionine, lysine, threonine, lysine plus threonine, S-adenosylmethionine or S-methylmethionine sulfonium to the assay mixture. Parallel studies of P and A showed that P activity responded to growth conditions identically to C, whereas A activity remained unaffected. Lemna extracts did not catalyze lanthionine formation from O-acetylserine and cysteine. Estimates of kinetic constants for the 3 enzyme activities indicate that A has much higher activity and affinity for sulfide than P. Apparently methionine or 1 of its products regulates the amount of active C in Lemna; P and C are probably activities of 1 enzyme that has low specificity for its S-containing substrate and A is a separate enzyme. The relatively high activity and affinity for sulfide of A provides an explanation in molecular terms for transsulfuration, and not direct sulfhydration, being the dominant pathway for homocysteine biosynthesis.