Four isozymes, I', II'a, II'b and III’ of lipoxygenase (EC 1.13.11.12) from Canola (Brassica napus, cv Westar) seed were purified by successive chromatography on ion‐exchange and size‐exclusion columns using a Fast Protein Liquid Chromatograph (FPLC). The homogeneity of each isozyme was demonstrated by a single protein band on SDS‐polyacrylamide gel electrophoresis. The molecular weights of isozymes I', II'a, II'b and III’ were 72, 000, 106, 000, 78, 000 and 62, 000, respectively. The optimum pH for lipoxygenase activity was 6.5 for isozyme I’ and 6.0 for isozymes II'b, II'b and III'. Apparent Km value for isozymes I', II'a, II'b and III’ were 5.5 × 10−4 M, 3.4 × 10−4 M, 4.0 × 10−4 M and 3.8 × 10−4 M, respectively. Isozyme I’ displayed preferential activity towards monolinoleate and dilinoleate, while isozyme II'a demonstrated preferential activity towards dilinoleate followed by mono‐ and trilinoleate. No enzymatic activity was observed with both isozymes I’ and II'a toward free linoleic acid. Isozyme II'b showed activity towards free linoleate as well as mono‐, di‐ and trilinoleate. Isozyme III’ showed preferential activity towards free linoleate. The activity of isozymes I’ and II'a was inhibited completely by the addition of 10 mM and 4 mM KCN, respectively, while the addition of 3 mM and 10 mM KCN to isozymes II'b and III', respectively, increased activity by approximately 20%.
