Enzymatic synthesis of the neuroexcitatory amino acid quisqualic acid by cysteine synthase

Abstract

Purification of cysteine synthase from the leaves of Quisqualis indica var. villosa reveals the presence of two forms of this enzyme, separated by chromatography on DEAE-Sephadex A-50. Isoenzyme A was purified 10 000-fold and had a specific activity of 10.8 U/mg protein. Isoenzyme B was purified 460-fold with a specific activity of 0.49 U/mg protein. Both isoenzymes have the same M,s (58 000) and dissociate into identical subunits ( M r 29 000). The K m value of isoenzyme A is 1.9 mM for O-acetyl- L-serine and 59 μM for sulphide, while that of isoenzyme B is 7.1 mM for O-acetyl- L-serine and 4.0 mM for 3,5-dioxo-1,2,4-oxadiazolidine. Both isoenzymes catalyse the formation of cysteine from O-acetyl- L-serine and hydrogen sulphide, but only isoenzyme B catalyses the formation of L-quisqualic acid. Other significant differences occur in the substrate specificity of the two isoenzymes. Some properties of the purified cysteine synthase isoenzymes are also described.