A comparative study was performed on some enzymatic properties of myrosinase isoenzymes (thioglucoside glucohydrolases, EC 18.104.22.168) from white mustard seed (Sinapis alba) and rapeseed Brassica napus). The investigation comprised the determination of pH optima, temperature maxima and the enzymatic stabilities of the isoenzymes, when temperature, pH, buffer and time were varied. The effect of ascorbic acid on the kinetic constants Km and V was found to be different for the isoenzymes. Comparative studies were also made of the activities of the isoenzymes on various glucosinolates, and the myrosinase activities in a number of cultivars of B. napus, Brassica campestris and S. alba. The results indicate that there are both physicochemical and enzymatic differences between myrosinase isoenzymes and that a large amounty of variance exists. They are, however, likely to have the same genetic origin. No significant variation in substrate specificity was found among the isoenzymes.
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