Cleavage of l-cystine by soluble enzyme preparations from Brassica species

Abstract

The decomposition of l-cystine to pyruvate and the persulfide, thiocysteine, is catalyzed by soluble enzymes obtained from tissues of brassica species. the enzyme, called trivially cystine lyase, requires pyridoxal-5′-PO 4 for activity; however, the stimulation of activity by exogenous cofactor is dependent on the species. Partial purification of the enzyme from the root of B. napobrassica (rutabaga) was accomplished. The rutabaga lyase was completely inactive in the absence of added pyridoxal-5′-PO 4. The PH optimum was between 8.5 And 8.9. The lyase was specific for the l-configuration of the amino acid and was only slightly inhibited by equimolar concentrations of The D-Form. The k m for l-cystine was 1 mm and was 0.5 μm for pyridoxal-5′-PO 4. There was a severe competitive inhibition by thiol compounds such as cysteine and reduced glutathione. The k i for both of these compounds was 1.5 × 10 −4 m. The enzyme preparation was capable of utilizing s-Methyl- l-cysteine sulfoxide and l-cysteine- s-Sulfate as substrates. It was completely inactive toward l-cystathionine and dl-allocystathionine. The stoichiometry of the reaction was one mole of thiocysteine and one mole of pyruvate produced per mole of cystine.