Cystine lyase degrades L-cystine by a al-elimination to form cysteine persulide, pyruvate, and ammonia. This enzyme is common in Brassica sp. and has been purified to homogeneity from extracts of broccoli (Brassica okeracea var botrytis) buds. Two isozymes were separated on DEAE-Fractogel columns and the first peak, cystine lyase I further purified to homogeneity. The purified enzyme had a narrow range of substrate specificity with L-cystine and S-alkyl-L-cysteine sulfoxides being the primary substrates. The K,. for L-cystine was 1.9 millimolar and for S-ethyl-L-cysteine sulfoxide was 15.6 millimolar, suggesting that L-cystine would be prefeffed in vivo. Using gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis the molecular weight of the holoenzyme was estimated as 152,000 composed of subunits of approximately 49,000. This strongly suggests the native enzyme is a trimer. The presence of carbohydrate in the native enzyme was detected at the level of 5.8% on a weight basis. Except for the ability to utilize Lcystine as a substrate there are many similarities between cystine lyase I and the alliin lyase of onion (Allium cepa).This enzyme has been purified to homogeneity from garlic (8,14) and onion (17) and found in a number of Brassica sp. (10). The garlic and onion enzymes are specific for S-alkyl-L-cysteine sulfoxides, however the Brassica preparations studied to date (1,6,11,12) also can cleave L-cystine according to reaction 2: S -s H2C +