Myrosinase activity in soil extracts

Myrosinase (beta-thioglucoside glucohydrolase; EC 3.2.3.1) catalyzes the hydrolysis of glucosinolates to form a variety of potential allelochemicals. Although these allelochemicals may exert an influence on soil-borne organisms, the extracellular preservation of plant-derived myrosinase in soil has not been determined. Soil samples from a held of rapeseed (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 enzymatic activity using sinigrin (allyl glucosinolate) as a substrate. Myrosinase activity was evaluated by monitoring the hydrolysis product, allyl isothiocyanate, using gas chromatography. Reliability and limitations of the assay were verified with autoclaved samples amended with known amounts of enzyme or allyl isothiocyanate. Extracts of soil samples 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 myrosinase activity equivalent to a myrosinase concentration of approximately 5 mu g kg(-1) of soil. The presence of extracellular myrosinase in soil indicates that glucosinolate hydrolysis in the rhizosphere of Brassica spp. and allelochemical impacts on organisms within the rhizosphere are possible.