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Abbreviations: AITC, allyl isothiocyanate; MA, mercapturic acids; AMA, allyl MA; SFMA, sulforaphane MA
Epidemiological studies suggest that the cancer-protective effects of brassica vegetables may be partly related to their content of glucosinolates (van Poppel et al. 1999). During the preparation and cooking of brassica, followed by their ingestion and digestion in the upper alimentary tract, glucosinolates are hydrolysed to thiohydroxamate-O-sulfonate by a plant thioglucosidase, myrosinase (EC 3.2.1.147), which is released from ruptured plant cells. Thiohydroxamate-O-sulfonate is then immediately converted to different classes of metabolites depending on the properties of the hydrolysis medium (Fenwick & Heaney, 1983). A further stage of hydrolysis of glucosinolates occurs in the colon under the action of colonic microbiota (Elfoul et al. 2001; Krul et al. 2002). Isothiocyanates, produced from hydrolysis of glucosinolates at a neutral pH, are one of the main metabolites implicated in the cancer-protective effects of brassica (Zhang et al. 2006).
An understanding of the digestive and absorptive fate of dietary glucosinolates and their metabolites has emerged mostly from mechanistic studies in animal models (Brüsewitz et al. 1977; Mennicke et al. 1983; Duncan et al. 1997; Elfoul et al. 2001). A few studies, involving human subjects, have used urinary biomarkers to demonstrate the absorption of isothiocyanates in vivo after the intake of glucosinolates from brassica vegetables (Getahun & Chung, 1999; Conaway et al. 2000; Shapiro et al. 2001; Rouzaud et al. 2004). After their absorption, isothiocyanates are metabolised by the mercapturic acid pathway in vivo. Isothiocyanates initially form conjugates with glutathione, then undergo enzymatic modifications, and are excreted in urine as their corresponding N-acetylcysteine conjugates or mercapturic acids (MA) (Brüsewitz et al. 1977). Urinary excretion of MA of isothiocyanates therefore reflects their production and uptake in vivo.
The production of isothiocyanates following the ingestion of brassica is influenced by residual glucosinolate concentration and plant myrosinase activity in the ingested vegetable. However, brassica vegetables are normally consumed as part of a meal and the digestive fate of glucosinolates may be further influenced by the composition of the meal matrix.
Sulforaphane, the alkyl isothiocyanate in broccoli, is produced from hydrolysis of its alkyl glucosinolate precursor, glucoraphanin. The aim of the present study was to determine the effect...