There has been significant interest in the glucosinolate-myrosinase system in plants of the Brassicaceae due to accumulating evidence that some glucosinolate degradation products are anticarcinogenic and/or suppressive to plant pathogens. Because glucosinolate hydrolysis is catalyzed by endogenous myrosinase, characterization of myrosinase activity is important for elucidating the potential bioactivity of crop glucosinolates. We measured the specific activity in citrate-phosphate buffer extracts across the pH range 4.5–6.5 of two cultivars each of five Brassica groups grown during two fall and two spring seasons. Specific activity in two kale cultivars was highly variable, but tended to have highest activity from pH 5.0–6.0. In both cauliflower cultivars from Fall 2000, Fall 2001, and Spring 2002, optimal pH was around pH 6.0. In Spring 2000, however, specific activity was highest at pH 5.0. Maximum specific activity in both cabbage cultivars occurred in the pH range 5.5–6.0 in Fall 2000, Fall 2001, and Spring 2002. In Spring 2000, specific activity in `Red Acre' cabbage was uniform across the range pH 4.5–5.5 and maximum specific activity was at pH 5.0 for `Early Round Dutch' cabbage. Both brussels sprouts cultivars had pH maxima around pH 5.5–6.0 and significantly lower activity at pH 4.5. Specific activity in broccoli was much like that of cauliflower in that highest activity occurred around pH 5.5–6.0 in Fall 2000, Fall 2001, and Spring 2002, but in Spring 2000, maximum activity was at pH 5.0. These results indicate that in most cases, pH optima were in the range 5.5–6.0, but varied somewhat with season and genotype.
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