Non-identity of cystine lyase with β-cystathionase in turnip roots

Abstract

An active preparation of cystine lyase (EC 4.4.1.-) was prepared from turnip roots and its substrate specificity examined. Only L-cysteine, cysteine-S-SO 3, 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.