Purification and Characterization of Cystine Lyase a from Broccoli Inflorescence.

One of the three isoforms of an enzyme degrading L-cystine was purified to homogeneity from broccoli (Brassica oleracea var. italica) inflorescences, with use of a sensitive assay based on derivatization of a reaction product with monobromobimane. The reaction product with a thiol group was found to be thiocysteine from results of liquid chromatography-mass spectrometry and high-resolution mass spectrometry. Pyruvate was also a reaction product, formed in equimolar amounts. The purified enzyme catalyzed β-elimination of L-cystine to yield thiocysteine, pyruvate and possibly ammonia, so it was cystine lyase a. L-Cystine but not D-cystine was a substrate of the enzyme. S-Methyl L-cysteine sulfoxide and S-ethyl L-cysteine sulfoxide were substrates but were less suitable than L-cystine. L- and D-cysteine and also cystathionine were not substrates. The purified enzyme (Mr 186,000) was composed of four identical subunits (Mr 45,000) and was pyridoxal 5'-phosphate-dependent.