BIOSYNTHESIS OF BETA-(ISOXAZOLIN-5-ON-2-YL)-L-ALANINE BY CYSTEINE SYNTHASE IN LATHYRUS-SATIVUS

Two different forms of cysteine synthase (CSase) were purified from th e grass pea (Lathyrus sativus), being separated by chromatography on D EAE-Sepharose Fast Flow or EAH-Sepharose 4B. The native isoenzyme A wa s a dimer with a subunit M(r) of 35 000, whereas the isoenzyme B was a lso a dimer with a subunit M(r) of 39 000. The K(m) values of isoenzym es A and B were 6.1 and 7.2 mM for O-acetyl-L-serine (OAS), respective ly. Data on the substrate specificity show that both isoenzymes cataly se the formation of beta-(isoxazolin-5-on-2-yl)-L-alanine (BIA), the b iosynthetic precursor of the neurotoxin beta-N-oxalyl-L-alpha,beta-dia minopropionic acid (ODAP), and some other heterocyclic beta-substitute d alanines from OAS as an additional catalytic activity. Antiserum pro duced from purified spinach CSase isoenzyme A was cross-reactive with Lathyrus isoenzyme A, but less with isoenzyme B. Several properties, i ncluding the amino acid compositions of the purified enzymes, and the intracellular localization of these enzymes in the grass pea and pea ( Pisum sativum) are also described.