salmonella typhimurium/enzymol.

Cysteine synthetase from Salmonella typhimurium LT-2 displays a saturation curve for sulfide identical to that obtained with uncomplexed O-acetylserine sulfhydrylase, indicating substrate inhibition with a K_m of 0.1 ± 0.017 mm and a K₁ of 0.303 ± 0.194 mm. With both l-serine and acetyl CoA, however, cysteine synthetase exhibits two intermediary plateaus in the respective saturation curves. The time course of cysteine synthetase activity when the reaction is started by adding enzyme displays a pronounced lag phase. This lag is explained as being due to the buildup of a sufficient concentration of O-acetyl-l-serine to permit binding to O-acetylserine sulfhydrylase. This conclusion is substantiated by the fact that plots of $\text{1}\text{τ}$ against concentrations of both l-serine and acetyl CoA reflect the saturation curves for these substrates. In addition, the incubation of the complex with l-serine and acetyl CoA results in the accumulation of the intermediate products of the reaction sequence, CoA and O-acetyl-l-serine. Dissociation of the multienzyme complex under these conditions was ruled out by Sephadex G-200 chromatography of the complex after incubation with assay levels of the substrates of the reaction. Aggregation of cysteine synthetase was detected using disc gel electrophoresis and confirms earlier reports Kredich, N. M., and Tomkins, G. M. (1966) J. Biol. Chem.241, 4955–4965]. Aggregation of O-acetylserine sulfhydrylase was also detected using the same technique.