As a cytosolic transcription factor, the aryl hydrocarbon (Ah) receptor is involved in several patho- physiological events leading to immunosuppression and cancer; hence antagonists of the Ah receptor may possess chemoprevention properties. It is known to modulate carcinogen-metabolising enzymes, for instance the CYP1 family of cytochromes P450 and quinone reductase, both important in the biotransformation of many chemical carcinogens via regulating phase I and phase II enzyme systems. Utilising chemically-activated luciferase expression (CALUX) assay it was revealed that intact glucosinolates, glucoraphanin and glucoerucin, isolated from Brassica oleracea L. var. acephala sabellica and Eruca sativa ripe seeds, respectively, are such antagonists. Both glucosinolates were poor ligands for the Ah receptor; however, they effectively antagonised activation of the receptor by the avid ligand benzo[a]pyrene. Indeed, intact glucosinolate glucoraphanin was a more potent antagonist to the receptor than glucoerucin. It can be concluded that both glucosinolates effectively act as antagonists for the Ah receptor, and this may contribute to their established chemoprevention potency.
Glucoraphanin is the main glucosinolate found in broccoli and other cruciferous vegetables (Brassicaceae). The objective of the study was to evaluate whether glucoraphanin and its breakdown product sulforaphane, are potent modulators of various phase I and phase II enzymes involved in carcinogen-metabolising enzyme systems in vitro. The glucosinolate glucoraphanin was isolated from cruciferous vegetables and exposed to human hepatoma cell line HepG2 at various concentrations (0-25 μM) for 24 hours. Glucoraphanin at higher concentration (25 μM) decreased dealkylation of methoxyresorufin, a marker for cytochrome P4501 activity; supplementation of the incubation medium with myrosinase (0.018 U), the enzyme that converts glucosinolate to its corresponding isothiocyanate, showed minimal induction in this enzyme activity at concentration 10 μM. Quinone reductase and glutathione S-transferase activities were unaffected by this glucosinolate; however, supplementation of the incubation medium with myrosinase elevated quinone reductase activity. It may be inferred that the breakdown product of glucoraphanin, in this case sulforaphane, is superior than its precursor in modulating carcinogen- metabolising enzyme systems in vitro and this is likely to impact on the chemopreventive activity linked to cruciferous vegetable consumption.