Shapiro T A, Fahey J W, Wade K L, Stephenson K K, Talalay P
Department of Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
Cancer Epidemiol Biomarkers Prev. 1998 Dec;7(12):1091-100.
Isothiocyanates and their naturally occurring glucosinolate precursors are widely consumed as part of a diet rich in cruciferous vegetables. When plant cells are damaged, glucosinolates are released and converted to isothiocyanates by the enzyme myrosinase. Many isothiocyanates inhibit the neoplastic effects of various carcinogens at a number of organ sites. Consequently, these agents are attracting attention as potential chemoprotectors against cancer. As a prerequisite to understanding the mechanism of the protective effects of these compounds, which is thought to involve the modulation of carcinogen metabolism by the induction of phase 2 detoxication enzymes and the inhibition of phase 1 carcinogen-activating enzymes, we examined the fate of ingested isothiocyanates and glucosinolates in humans. Recently developed novel methods for quantifying isothiocyanates (and glucosinolates after their quantitative conversion to isothiocyanates by purified myrosinase) and their urinary metabolites (largely dithiocarbamates) have made possible a detailed examination of the fates of isothiocyanates and glucosinolates of dietary crucifers. In a series of studies in normal volunteers, we made these findings. First, in nonsmokers, urinary dithiocarbamates were detected only after the consumption of cruciferous vegetables and condiments rich in isothiocyanates and/or glucosinolates. In sharp contrast, the consumption of noncrucifers (corn, tomatoes, green beans, and carrots) did not lead to the excretion of dithiocarbamates. Moreover, the quantities of dithiocarbamates excreted were related to the glucosinolate/isothiocyanate profiles of the cruciferous vegetables administered (kale, broccoli, green cabbage, and turnip roots). Second, eating prepared horseradish containing graded doses of isothiocyanates (12.3-74 micromol; mostly allyl isothiocyanate) led to a rapid excretion of proportionate amounts (42-44%) of urinary dithiocarbamates with first-order kinetics. The ingestion of broccoli in which myrosinase had been heat-inactivated also led to proportionate but low (10-20%) recoveries of urinary dithiocarbamates. Broccoli samples subsequently treated with myrosinase to produce the cognate isothiocyanates were much more completely (47%) converted to dithiocarbamates. Finally, when bowel microflora were reduced by mechanical cleansing and antibiotics, the conversion of glucosinolates became negligible. These results establish that humans convert substantial amounts of isothiocyanates and glucosinolates to urinary dithiocarbamates that can be easily quantified, thus paving the way for meaningful studies of phase 2 enzyme induction in humans.
异硫氰酸盐及其天然存在的硫代葡萄糖苷前体作为富含十字花科蔬菜的饮食的一部分被广泛食用。当植物细胞受损时，硫代葡萄糖苷被释放，并被黑芥子酶转化为异硫氰酸盐。许多异硫氰酸盐在多个器官部位抑制各种致癌物的致癌作用。因此，这些物质作为潜在的抗癌化学保护剂正受到关注。作为理解这些化合物保护作用机制的前提，人们认为其保护作用机制涉及通过诱导Ⅱ相解毒酶来调节致癌物代谢以及抑制Ⅰ相致癌物激活酶，我们研究了人体摄入的异硫氰酸盐和硫代葡萄糖苷的去向。最近开发的用于定量异硫氰酸盐（以及通过纯化的黑芥子酶将硫代葡萄糖苷定量转化为异硫氰酸盐后）及其尿液代谢物（主要是二硫代氨基甲酸盐）的新方法，使得详细研究十字花科蔬菜中异硫氰酸盐和硫代葡萄糖苷的去向成为可能。在一系列针对正常志愿者的研究中，我们得到了以下发现。首先，在不吸烟者中，只有在食用富含异硫氰酸盐和/或硫代葡萄糖苷的十字花科蔬菜和调味品后，才会在尿液中检测到二硫代氨基甲酸盐。与之形成鲜明对比的是，食用非十字花科蔬菜（玉米、西红柿、绿豆和胡萝卜）不会导致二硫代氨基甲酸盐的排泄。此外，排泄的二硫代氨基甲酸盐的量与所摄入十字花科蔬菜（羽衣甘蓝、西兰花、绿甘蓝和芜菁根）的硫代葡萄糖苷/异硫氰酸盐谱有关。其次，食用含有不同剂量异硫氰酸盐（12.3 - 74微摩尔；主要是烯丙基异硫氰酸盐）的辣根会导致尿液中二硫代氨基甲酸盐按比例快速排泄（42 - 44%），排泄动力学符合一级动力学。食用黑芥子酶已被热灭活的西兰花也会导致尿液中二硫代氨基甲酸盐按比例但较低（10 - 20%）的回收率。随后用黑芥子酶处理西兰花样品以产生相应的异硫氰酸盐，其转化为二硫代氨基甲酸盐的程度要高得多（47%）。最后，当通过机械清洁和抗生素减少肠道微生物群时，硫代葡萄糖苷的转化变得微不足道。这些结果表明，人体会将大量的异硫氰酸盐和硫代葡萄糖苷转化为易于定量的尿液二硫代氨基甲酸盐，从而为在人体中进行有意义的Ⅱ相酶诱导研究铺平了道路。
