Quantitation of glucosinolates in vegetables consumed in Singapore: Relationship to metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in smokers.

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Lung cancer is the most common cause of cancer death in the world. Vegetable consumption, including cruciferous vegetables, decreases lung cancer risk, but the mechanisms are poorly understood. Glucosinolates are unique constituents of cruciferous vegetables and these compounds are found in milligram quantities per normal servings. When the cruciferous vegetables are consumed, the glucosinolates are hydrolyzed by myrosinase releasing a variety of hydrolysis products such as isothiocyanates, nitriles, thiocyanates, and indole-3-carbinols. Indole-3-carbinol (I3C), the hydrolysis product of glucobrassicin, inhibits lung tumorigenesis in mice treated with the tobacco-specific lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Singapore Chinese consume a diet rich in cruciferous vegetables. In this study, 84 Singapore Chinese smokers provided urine samples and were interviewed about dietary habits using a structured questionnaire, which included questions on consumption of nine commonly consumed cruciferous vegetables. The vegetables- watercress, pak choi, cabbage, cauliflower, wong nga pak, kai choi, kai lan, choi sum, and broccoli- were purchased in Singapore markets at three different times of the year and analyzed for glucosinolates. Extracts of the vegetables and the cooking water from the boiled vegetables were desulphated by sulfatase and analyzed by HPLC. The desulphated glucosinolates were identified by retention time and comparison of UV spectra to those of standards. Glucobrassicans accounted for 70.0-93.2% of all glucosinolates in these vegetables. There was a significant correlation (P=0.01) between increased consumption of glucobrassicans and decreased levels of the NNK metabolite NNAL in urine after adjustment for number of cigarettes smoked per day. Similar trends were observed for NNAL-glucuronides (P=0.08) and total NNAL (P=0.03). The results are consistent with those of previous studies, which demonstrate that I3C decreases levels of urinary NNAL in smokers, probably by inducing hepatic metabolism of NNK. This may be one mechanism by which cruciferous vegetables protect against lung cancer in smokers.