MYROSINASE ACTIVITY IN SOIL EXTRACTS

Myrosinase (beta-thioglucoside glucohydrolase; EC 3.2.3.1) catalyzes t he hydrolysis of glucosinolates to form a variety of potential alleloc hemicals. Although these allelochemicals may exert an influence on soi l-borne organisms, the extracellular preservation of plant-derived myr osinase in soil has not been determined. Soil samples from a held of r apeseed (Brassica napus L.) and from a pasture were extracted with a 0 .1 M mixture of K2HPO4 and disodium ethylenedinitrilotetraacetate (pH 7.9). Crude extracts were purified with dialysis and assayed for enzym atic activity using sinigrin (allyl glucosinolate) as a substrate. Myr osinase activity was evaluated by monitoring the hydrolysis product, a llyl isothiocyanate, using gas chromatography. Reliability and limitat ions of the assay were verified with autoclaved samples amended with k nown amounts of enzyme or allyl isothiocyanate. Extracts of soil sampl es from a rapeseed field showed myrosinase activity, whereas extracts from a pasture soil showed no detectable activity. Highest myrosinase activity was detected in soil sampled directly from rapeseed rows, an activity equivalent to a myrosinase concentration of approximately 20 mu g kg(-1) soil. Soil sampled between rapeseed rows had a lower myros inase activity equivalent to a myrosinase concentration of approximate ly 5 mu g kg(-1) of soil. The presence of extracellular myrosinase in soil indicates that glucosinolate hydrolysis in the rhizosphere of Bra ssica spp. and allelochemical impacts on organisms within the rhizosph ere are possible.