Broccoli & Signalling Pathways thioglucosidases in the colon (Figure 1) [13]. These ITCs do not have the pungent flavour qualities associated with other dietary ITCs, such as those from mustards, rockets and watercress. SF and IB are passively absorbed by enterocytes, conjugated with glutathione and transported into the systemic circulation to be metabolized via the mercapturic acid pathway and excreted predominantly as N-acetylcysteine conjugates in the urine [14]. We previously demonstrated that following broccoli consumption, 45% of SF in the plasma occurs as free SF, as opposed to thiol conjugates, and that the peak concentration of SF and its thiol conjugates is less than 2 mM, falling to low (nM) levels within a few hours [14]. It was also shown that GSTM1 individuals excrete a higher proportion of SF via mercapturic acid metabolism than GSTM1 positive individuals, and it was speculated that the remaining SF may be metabolized via an unknown pathway, and that this may account for the anticarcinogenic activity of broccoli. Cell and animal studies have shown that SF is a potent inducer of phase 2 enzyme gene transcription, and, at higher concentra- tions, of cell cycle arrest and apoptosis, all consistent with anticarcinogenic activity [15]. However, these phenomena occur when cultured cells are exposed to considerably higher levels of SF (i.e. 10–100 mM for 24 h) than those found transiently in plasma Figure 1. Metabolism of 4-methylsulphinylbutyl glucosinolate and sulforaphane. Upon entry into enterocytes sulforaphane (SF) is rapidly conjugated to glutathione, exported into the systemic circulation and metabolized through the mercapturic acid pathway. Within the low glutathione environment of the plasma the SF-glutathione conjugate may be cleaved, possibly mediated by GSTM1, leading to circulation of free SF in the plasma. This free SF can modify plasma proteins including signalling molecules, such as TGFb, EGF and insulin. doi:10.1371/journal.pone.0002568.g001 PLoS ONE | www.plosone.org 2 July 2008 | Volume 3 | Issue 7 | e2568