Digestive metabolism of glucosinolates : a novel approach using urinary markers for estimating the release of glucosinolate breakdown products in the gastro-intestinal tract of mammals
Glucosinolates have been implicated as a mediator of the cancer-protective properties of cruciferous vegetables. Enzymatic hydrolysis of glucosinolates by plant or microbial myrosinase yields a range of metabolites including beneficial isothiocyanates. Little is known about the fate of glucosinolates after their ingestion. Using urinary end-products of metabolism as markers, measurement of the production of isothiocyanates in the digestive tract of monogastric animals has been achieved. Initially, a range of isothiocyanates were administered to rats and their excretion as mercapturic acids was quantified. Relative recovery of different isothiocyanates was found to be consistent and predictable, allowing the use of artificial isothiocyanates as recovery standards in subsequent experiments. Subsequently, the relative influence of plant and bacterial myrosinase on isothiocyanate production was quantified in rats. A proportion of 0.80 (s. e. m. 0.076) of benzyl glucosinolate was hydrolysed to isothiocyanate by plant myrosinase. In the presence of both plant and microbial activity,, the proportion of benzyl isothiocyanate release was significantly decreased (0.50 s. e. m. 0.046, p<0.01) suggesting microbial breakdown of isothiocyanates. The approach, adapted for use with human subjects showed that the proportions of allyl isothiocyanate measured after ingestion of raw and cooked cabbage were 0.37 (s. e. m. 0.045) and 0.53 (s. e. m. 0.134) respectively in healthy male volunteers. A further experiment with rats established that isothiocyanate uptake in the distal digestive tract was significantly less than in the proximal intestine (0.12 s. e. m. 0.017 and 0.48 s. e. m. 0.029 respectively), suggesting a potential underestimation of isothiocyanate release in the distal digestive tract when using urinary markers. Finally, enhancement of bacterial fermentation by addition of inulin to the diet had little influence on isothiocyanate production in the gut. The findings suggested that the formation of the cancer-protective isothiocyanates was significant, in vivo, thereby strengthening the evidence for a beneficial effect of cruciferous vegetables for health. The newly developed method opens up possibilities of concurrently exploring the digestive fate of isothiocyanates and the toxicity of carcinogenic compounds.