C-S lyase activities in leaves of crucifers and non-crucifers, and the characterization of three classes of C-S lyase activities from oilseed rape (Brassica napus L.)

C-S lyases in plants are involved in primary and secondary metabolism, and in glucosinolate-containing species may be involved in glucosinolate biosyn thesis. Extracts from oilseed rape (Brassica napus) leaves were assayed for several C-S lyase activities. Pour activities [using L-cystine, L-cystathi onine, S-(2-benzothiazolyl)-L-Cys (SBC) and S-benzyl-L-Cys] were investigat ed in detail. All are developmentally regulated (highest in youngest leaves ), and differentially inhibited by iodoacetamide, N-ethylmaleimide (NEM) an d ethylenediaminetetraacetic acid (EDTA). Thermal stabilities and pH optima were also distinct, Competitive inhibition of the SEC lyase activity with a variety of sulphur-containing compounds indicated that cystine lyase cont ributes to SEC degradation, and this enzyme may cleave a wide range of comp ounds, both aliphatic and aromatic, but other 'SBC lyases' were also presen t, Putative aromatic glucosinolate intermediates were cleaved by the rape e nzymes. Developmental and biochemical studies indicate that at least three classes of C-S lyase activity are present in rape leaves: cystathionine bet a-lyase, cystine lyase and a group of relatively non-specific lyases, C-S l yase. preparations from other glucosinolate- and non-glucosinolate-containi ng species were capable of cleaving a number of aliphatic and aromatic conj ugates, The highest activities were detected in glucosinolate-containing sp ecies and Allium cepa (onion). C-S lyase activities in non-glucosinolate-co ntaining species (tobacco, Nicotiana tabacum, and barley, Hordeum vulgare) were much lower.