Biosynthetic mechanism and physiological role of heterocyclic ß-substituted alanines in higher plants

Abstract

Purification of cysteine synthase from the seedlings of Leucaena leucocephala reveals the presence of two forms of this enzyme, separated by chromatography on DEAE-Sephadex A-50. Both isoenzymes have the same Mr of 64,000 consisting of two identical subunits each. The subunits contain one molecule of pyridoxal 5′-phosphate. Cysteine synthases from L. leucocephala catalyzed the formation of some heterocyclic ß-substituted alanines such as ß-Cpyrazol-l-yl)-L-alanine from O-acetyl-L-serine as an additional catalytic activity, but only isoenzyme B catalyzed the formation of L-mimosine in the same manner as the formation of L-quisqualic acid in Quisqualis indica var. villosa. The properties of the cysteine synthase isoenzymes purified in this study suggest that all cysteine synthases in higher plants can be multifunctional enzymes that are involved in the biosynthesis of ß-substituted alanines, and that these enzymes may have different substrate specificities. The physiological role of these enzymes in higher plants is also described.