The Stoichiometry and Kinetics of the Inducible Cysteine Desulfhydrase from Salmonella typhimurium

Studies using highly purified cysteine desulfhydrase from Salmonella typhimurium reveal that only a small fraction of the cysteine utilized by the enzyme appears as pyruvate. The isolation of 2-methyl-2, 4-thiazolidinedicarboxylic acid from reaction mixtures offers an explanation for this unusual stoichiometry. The relative amounts of pyruvate and thiazolidine produced during a reaction depend upon the cysteine concentration, pH, and the presence of a protein termed Fraction B, which prevents the formation of the thiazolidine. We propose that 2-aminoacrylate may be an intermediate in the formation of 2-methyl-2, 4-thiazolidinedicarboxylic acid. Substrate velocity curves for cysteine desulfhydrase reveal positive cooperativity with an n value of 1.9 and a Km, for l-cysteine of 0.17 to 0.21 mm. The product, sulfide, inhibits the reaction with a Ki of 0.010 mm. Sulfide inhibition is of the linear competitive type at high cysteine concentrations, but it becomes nonlinear and more pronounced at low cysteine concentrations.