GSTM1, GSTT1, GSTP1, and GSTA1 polymorphisms and urinary isothiocyanate metabolites following broccoli consumption in humans

Isothiocyanates (ITC) are potentially anticarcinogenic phytochemicals formed from the metabolism of glucosinolates and are found in cruciferous vegetables as well as a select number of other foods. ITC are both substrates for and inducers of glutathione S-transferase (GST) phase 11 metabolizing enzymes involved in carcinogen detoxification as well as effectors of phase I pathways. Previous studies report mixed results on the interaction between cruciferous vegetable intake, GST polymorphisms, and risk of cancer. We conducted a study of 114 healthy human subjects between 18 and 50 y of age to examine the biologic mechanism underlying the associations, specifically, to assess whether GST genotype is associated with urinary ITC metabolites following a known dose of broccoli. After 48 h of abstaining from all sources of glucosinolates, participants provided a blood sample, consumed 1 meal containing 2.5 g broccoli/kg body weight, and collected urine for 24 h. ITC metabolites were measured in the urine using a HPLC cyclocondensation assay. DNA was extracted from blood samples, and GSTM1 deletion, GSTT1 deletion, GSTP7 Ile705Val, and GSTA1*A/*B were genotyped by matrix-assisted laser desorption/ionization time-of-flight. A chi-square test was used to compare high and low ITC excretion levels across genotypes. ITC levels were regressed on genotype, adjusting for gender. There were no substantial differences in ITC levels among genotypes, either individually or in combination. Contrary to our hypothesis, a higher proportion of GSTM1 null individuals had high ITC excretion (62%) compared with the proportion of GSTM1 present with high ITC excretion (39%) (P = 0.03). These results are in agreement with another feeding study, and lend support to the idea of alternative routes of ITC metabolism.