The Cysteine Desulfhydrase of Salmonella typhimurium

The effects of cryolysis on the inducible regulatory enzyme, cysteine desulfhydrase from Salmonella typhimurium, were examined. The stoichiometry of product formation catalyzed by partially purified preparations changes from one sulfide to one pyruvate to as much as 1:0.3 as a function of slow freezing and thawing. The ratio of the value for Vmax of sulfide formation to the value for Vmax of pyruvate formation also changes. The order of substrate dependence (n from Hill plots) does not change when velocity is measured via sulfide formation but changes from 1.9 to as much as 3 when velocity is measured via pyruvate formation. Cryolyzed preparations apparently catalyze the abnormal decomposition of cysteine to alanine and sulfide instead of pyruvate, ammonia, and sulfide. Methionine and dithiothreitol prevent the changes that occur upon freeze-aging. Sephadex G-200 chromatography revealed the presence in crude extracts of a protein (P II) with a Kd three times greater than that of cysteine desulfhydrase, and which is capable of converting altered enzyme to native enzyme. The addition of P II to cryolyzed preparations restores the 1:1 stoichiometry of product formation, restores the native order of substrate dependence of pyruvate formation, and prevents the formation of alanine. This restorative protein is heat stable. It has thiopyruvate desulfurase activity and is induced in parallel with cysteine desulfhydrase. It is proposed that: (a) P II activity represents a catalytic subunit of cysteine desulfhydrase which displays thiopyruvate desulfurase activity instead of cysteine desulfhydrase activity; (b) the catalytic subunit is cold-labile when present in native enzyme but is stable to both heat and cold when in the free subunit form; (c) the cryolysis of native enzyme results in the formation of an altered enzyme containing an altered catalytic subunit. This altered enzyme catalyzes the abnormal production of alanine and sulfide from cysteine instead of pyruvate, ammonia, and sulfide, resulting in a decrease of the 1:1 stoichiometry of pyruvate and sulfide production; (d) the addition of P II to freeze-altered enzyme results in the exchange of good catalytic subunits for altered ones, regenerating the native enzyme.