Conversion of glucosinolates to isothiocyanates in humans after ingestion of cooked watercress.

Isothiocyanates (ITCs), major constituents of cruciferous vegetables, can inhibit tumorigenesis in rodents by modulating the metabolism of carcinogens. ITCs that occur as glucosinolates are released by myrosinase-mediated hydrolysis when raw vegetables are chopped or chewed. However, because cruciferous vegetables are commonly consumed by humans after being cooked, it is important to examine whether dietary glucosinolates are converted to ITCs after cooked cruciferous vegetables in which myrosinase is deactivated have been consumed. This information is useful for evaluating the potential role of ITCs in cruciferous vegetables in the protection against human cancers. A urinary marker, based on a cyclocondensation product formed by the reaction of ITCs and their conjugates with 1,2-benzenedithiol, was used to quantify the uptake of dietary ITCs in humans. At breakfast and lunch, nine volunteers consumed a total of 350 g of cooked watercress in which the myrosinase activity was completely deactivated. On the basis of the analysis of ITCs in the cooked watercress upon adding exogenous myrosinase, the amount of glucosinolates ingested by each subject was estimated to be 475 micromol. The 24-h urine samples showed that the total urinary excretion of ITC conjugates in the subjects ranged from 5.6 to 34.8 micromol, corresponding to 1.2-7.3% of the total amount ingested. On the basis of our previous results that approximately 50% of dietary ITCs were excreted in the urine as conjugates, these values represent the minimal in vivo conversion of glucosinolates to ITCs. For purposes of comparison, we carried out a second experiment in which 150 g of uncooked watercress were consumed. The percentage of urinary ITC conjugates excreted in this study ranged from 17.2 to 77.7% of the total ingested ITCs. These results indicate that glucosinolates are converted to ITCs in humans after ingestion of cooked watercress, in which the myrosinase has been completely inactivated. The extent of conversion, however, is considerably less than that after ingesting uncooked vegetables. Furthermore, upon incubation of the cooked watercress juice with fresh human feces under anaerobic conditions, approximately 18% of glucosinolates was hydrolyzed to ITCs in 2 h. These results suggest that the microflora in the intestinal tract are a likely source for the hydrolysis of glucosinolates to ITCs in humans.