Myrosinases, glucosinolates and transformation products in Brassica, Raphanus and Sinapis: Physico-chemical properties and activity of myrosinase isoenzymes

Myrosinases are glycoproteins composed by several minor protein subunits that occur as mixtures of isoenzymes. Myrosinases catalyse the hydrolysis of glucosinolates, producing a series of degradation products with biological activity. In the experiments presented in this paper, myrosinases were purified from seeds of S. alba, B. napus and two cultivars of R. sativus with different resistance to nematodes (Pegletta and Siletina). Three different molecular weight groups (500-600 kD (III), 280-350 kD (II) and 140-190 kD (I)) were found by G-200 gel filtration for B. napus and R. sativus myrosinases, whereas S. alba only showed the group of isoenzymes with the lowest molecular weight (I). By SDS-PAGE it was found that all isoenzyme groups were composed by 8-10 subunits of molecular weight ranging from 20-80 kD. In an assay with sinigrin the three isoenzyme groups within a single species showed different activity, the group with the lowest molecular weight (I) showing the lowest activity in both B. napus and R. sativus. The different activity against sinigrin showed by both cultivars of R. sativus may be related to their different resistance against nematodes although other mechanisms are also likely to be involved. The possibility for utilising Brassicaceous plants for their biological activity (fungicidal, nematocidal, nutritional, anticarcinogenic, etc...) requires the determination of the glucosinolate content of the original material, but it also requires the careful study of the myrosinase isoenzymes and the products resulting from their action on glucosinolates.