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Methionine Biosynthesis in Lemna

STUDIES ON THE REGULATION OF CYSTATHIONINE -SYNTHASE, O-PHOSPHOHOMOSERINE SULFHYDRYLASE, AND O-ACETYLSERINE SULFHYDRYLASE

National Institute of Mental Health, Bethesda, Maryland 20205, Laboratory of General and Comparative Biochemistry, Bethesda, Maryland 20205

Regulation of enzymes of methionine biosynthesis was investigated by measuring the specific activities of O-phosphohomoserine-dependent cystathionine -synthase, O-phosphohomoserine sulfhydrylase, and O-acetylserine sulfhydrylase in Lemna paucicostata Hegelm. 6746 grown under various conditions. For cystathionine -synthase, it was observed that (a) adding external methionine (2 µM) decreased specific activity to 15% of control, (b) blocking methionine synthesis with 0.05 µML-aminoethoxyvinylglycine or with 36 µM lysine plus 4 µM threonine (Datko, Mudd 1981 Plant Physiol 69: 1070-1076) caused a 2- to 3-fold increase in specific activity, and (c) 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 O-phosphohomoserine sulfhydrylase and O-acetylserine sulfhydrylase showed that O-phosphohomoserine sulfhydrylase activity responded to growth conditions identically to cystathionine -synthase activity, whereas O-acetylserine sulfhydrylase activity remained unaffected. Lemna extracts did not catalyze lanthionine formation from O-acetylserine and cysteine. Estimates of kinetic constants for the three enzyme activities indicate that O-acetylserine sulfhydrylase has much higher activity and affinity for sulfide than O-phosphohomoserine sulfhydrylase.

The results suggest that (a) methionine, or one of its products, regulates the amount of active cystathionine -synthase in Lemna, (b) O-phosphohomoserine sulfhydrylase and cystathionine -synthase are probably activities of one enzyme that has low specificity for its sulfur-containing substrate, and (c) O-acetylserine sulfhydrylase is a separate enzyme. The relatively high activity and affinity for sulfide of O-acetylserine sulfhydrylase provides an explanation in molecular terms for transsulfuration, and not direct sulfhydration, being the dominant pathway for homocysteine biosynthesis.

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