A urinary biomarker for uptake of dietary isothiocyanates in humans.

Isothiocyanates (ITCs) are a family of biologically active compounds that are distributed widely in cruciferous vegetables. Although studies in rodents have shown that these compounds are effective and versatile inhibitors of tumorigenesis, the role of dietary ITCs in the protection against human cancers remains to be established. A prerequisite of human studies is to develop an uptake biomarker for dietary ITCs. In this study, we describe a rapid high-performance liquid chromatography-based assay to measure the total ITC level in human urine. This assay is based on a previously described reaction of ITCs or their thiol conjugates with 1,2-benzenedithiol to yield a cyclocondensation product, 1,3-benzodithiole-2-thione, which then can be quantified by reverse phase high-performance liquid chromatography with UV detection. This new assay was validated by analyzing urine samples from 14 subjects who had consumed a known amount of watercress or brown mustard in a controlled experiment. The N-acetylcysteine conjugates of phenethyl ITC and allyl ITC from watercress and brown mustard, respectively, were quantified and compared with the results obtained from the current assay. Results of the two methods were highly correlated (r = 0.978), indicating the specificity of this new assay for dietary ITCs. The feasibility of this assay for population-based studies was examined using stored urine samples collected from nine participants of a prospective cohort study in Shanghai, China, who indicated that they were daily consumers of dark green vegetables. There was a 10-fold variation in urinary ITC contents among these samples, ranging from 0.7 to 7.0 micromol/g creatinine. These results show the potential use of this uptake biomarker in epidemiological studies to identify the role of dietary ITCs in modifying cancer risks in humans.