Cystine lyase

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.8], also referred to as /3-cystathionase and cystine lyase, catalyzes the hydrolysis of cystathionine to yield homocysteine, pyruvate, and ammonia. 

Enzyme Purification. Broccoli contained sufficient levels of peroxidase, lipase and cystine lyase to permit their isolation in the amounts needed. Only traces of lipoxygenase and catalase were present. Activities (units/g vegetable see assay methods below) were peroxidase, 220 lipase, 12 lyase, 0.26. Catalase was M) units/g in broccoli compared to 19 in English green peas lipoxygenase was 2 units/g in broccoli compared to 110 in English green peas. Peroxidase, lipase and cystine lyase were purified by... 

Aroma Analysis. Isolated peroxidase, lipase and cystine lyase were added singly and in various combinations to homogenates of blanched broccoli (free of enzyme activities) at the same activity level they were present in the unblanched broccoli. The samples were incubated at 30°C for three hours in a water bath and then cooked on hot plates for 20 min at 93°C. A teaspoon of sample was put into each of several wine glasses, the glasses covered and the samples incubated at room temperature (23 1°C) for at least 15 min before evaluation by the panel. 

Figure 1. Schematic diagram of the purification of peroxidase, cystine lyase and lipase from broccoli. PVP, polyvinylpyrrolidone PALP, pyridoxal-5 -phosphate.

As shown in Equations 1 and 2, cystine lyase produces cysteine, pyruvate, ammonia and sulfur from cystine. The cysteine is further acted upon to give pyruvate, hydrogen sulfide and ammonia. Hydrogen sulfide and ammonia formation probably account for the sulfhydryl compound and ammonia aromas detected in the unblanched sample. The pyruvic acid (at the pH of the samples) may account for the sour aroma detected in the unblanched sample, as well as samples treated with lyase and lipase separately. 

Alliinases, cystine lyases, histidine ammonia-lyase Xylose, mutase... 

Some aspects of the catabolism of cystine in Brassica species have already been discussed in Section V,A. Mazelis et al. (1982) and Hamamoto and Ma-zelis (1986) reported cystine lyase preparations from turnip roots and broccoli buds which were active against cystine and S-alkylcysteine sulfoxides but were inactive toward cystathionine. Hall and Smith (1983) purified two isoenzymes of cystine lyase from cabbage leaves, the more active of which was inactive toward cystathionine. Both isoenzymes supported the production of pyruvate from 5-methylcysteine sulfoxide at rates faster than those obtained with L-cys-tine. However, the values for these substrates were considerably greater than... 

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