NON-IDENTITY OF CYSTINE LYASE WTH &β-CYSTATHIONASE IN TURNIP ROOTS

Article abstract of DOI:10.1016/S0031-9422(00)82403-6

An active praparation of cystine lyase (EC 4.4.1-) was prepared from turnip roots and its substrate specificity examined.Only L-cysteine, cysteine-S-SO3, and the sulphoxides of L-djenkolic acid, S-methyl- and S-ethyl-L-cysteine were substrates.L-Cystathione, L-djenkolic acid, S-methyl- and S-ethyl-cysteines were not cleaved by this enzyme.The K_m for L-cystine was 1.3 mM and L-cystathionine acted as an effective competitive inhibitor with a K_i of 0.7 mM.After dialysis against 10 mM potassium phosphate buffer pH 7.5, added pyridoxal phosphate was absolutely necessary for activity.In addition a marked stimulation was observed in the presence of ammonium sulphate.The products of the reaction were cysteine persulphide, pyruvate and presumably ammonia.The persulphide was easily demonstrated by cleavage with CN^- to yield SCN^- under conditions in which elemental sulphur was unreactive. Key Word Index- Brassica rapa,; Cruciferae; turnip root; cystine lyase; β-cystathionase; cystine degradation.