Development and characteristics of myrosinase in Brassica napus during early seedling growth

Myrosinase (β‐thioglucoside glucohydroase, E. C. 3.2.3.1) proteins with different physical, but similar kinetic characteristics exist in oilseed rape ( Brassica napus L. cv. Bienvenu) seedlings. Two protein fractions have been described which are immunologically, and therefore likely to be structurally, related. Myrosinase I, a dimeric 156 kDa glycosylated protein was purified to apparent homogeneity, and polyclonal antibodies were raised against this protein. Myrosinase II, in comparison, was significantly less glycosylated. The native protein had a molecular weight of approximately 188 kDa, with subunit M r 's of mainly 62 kDa and also 68 kDa. Total ‘potential’enzyme activity (assayed in the presence of ascorbic acid activator) increased during early seedling growth. Immunoblot analysis of seedling proteins showed that this is directly related to an increase in the amount of myrosinase protein itself , predominantly myrosinase II proteins, which are not present in the dry seed. Myrosinase II protein is located exclusively in the cotyledons of 5‐day‐old seedlings, whilst myrosinase I is distributed throughout the seedling.