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 the grass pea (Lathyrus sativus), being separated by chromatography on DEAE-Sepharose Fast Flow or EAH-Sepharose 4B. The native isoenzyme A was a dimer with a subunit M(r) of 35 000, whereas the isoenzyme B was also a dimer with a subunit M(r) of 39 000. The K(m) values of isoenzymes A and B were 6.1 and 7.2 mM for O-acetyl-L-serine (OAS), respectively. Data on the substrate specificity show that both isoenzymes catalyse the formation of beta-(isoxazolin-5-on-2-yl)-L-alanine (BIA), the biosynthetic precursor of the neurotoxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (ODAP), and some other heterocyclic beta-substituted alanines from OAS as an additional catalytic activity. Antiserum produced from purified spinach CSase isoenzyme A was cross-reactive with Lathyrus isoenzyme A, but less with isoenzyme B. Several properties, including 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.