Our previous studies showed that phenethyl isothiocyanate (PEITC), a cruciferous vegetable constituent, inhibited the lung tumorigenesis induced by a potent tobacco-specific carcinogenic nitrosamine in animals. These results implicate dietary PEITC as a risk-reducing factor of lung cancers induced by smoking. To define the effect of dietary PEITC on human cancers, a method of measuring its uptake is needed. Since watercress is rich in gluconasturtiin, a glucosinolate precursor of PEITC, it was chosen to be the source of PEITC. Four individuals were asked to eat watercress as part of a breakfast meal, and 24-h urine samples were collected. A urinary metabolite was found, and its identity was confirmed as the N-acetylcysteine conjugate of PEITC by comparison with the synthetic standard using nuclear magnetic resonance and mass spectrometry. A dose-dependent excretion of this conjugate was observed. These results clearly showed that PEITC was released in the human body upon ingestion of watercress and suggest that the N-acetylcysteine conjugate of PEITC may be a useful marker for quantitating human exposure to this anticarcinogen as a tool for epidemiological investigations.

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