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 biosynthesis. Extracts from oilseed rape (Brassica napus) leaves were assayed for several C‐S lyase activities. Four activities [using L‐cystine, L‐cystathionine, S‐(2‐benzothiazolyl)‐L‐Cys (SBC) and S‐benzyl‐L‐Cys] were investigated in detail. All are developmentally regulated (highest in youngest leaves), and differentially inhibited by iodoacetamide, N‐ethylmaleimide (NEM) and ethylenediaminetetraacetic acid (EDTA). Thermal stabilities and pH optima were also distinct. Competitive inhibition of the SBC lyase activity with a variety of sulphur‐containing compounds indicated that cystine lyase contributes to SBC degradation, and this enzyme may cleave a wide range of compounds, both aliphatic and aromatic, but other ‘SBC lyases’ were also present. Putative aromatic glucosinolate intermediates were cleaved by the rape enzymes. Developmental and biochemical studies indicate that at least three classes of C‐S lyase activity are present in rape leaves: cystathionine β‐lyase, cystine lyase and a group of relatively non‐specific lyases. C‐S lyase preparations from other glucosinolate‐ and non‐glucosinolate‐containing species were capable of cleaving a number of aliphatic and aromatic conjugates. The highest activities were detected in glucosinolate‐containing species and Allium cepa (onion). C‐S lyase activities in non‐glucosinolate‐containing species (tobacco, Nicotiana tabacum, and barley, Hordeum vulgare) were much lower.