Sulfation and glucuronidation of benzo[a]pyrene-7,8-dihydrodiol in intestinal mucosa of channel catfish (Ictalurus punctatus)

Intestinal metabolism plays a significant role in the bioavailability of ingested environmental toxicants. In this study, the potential for first pass, phase 2 biotransformation of benzo[a]pyrene-7,8-dihydrodiol (BaP-7,8-diol) in intestinal mucosa was examined. Sulfotransferase and Uridine 5′-Diphospho-Glucuronyl-transferase activity were measured in cytosol, and microsomes respectively. Radiolabeled conjugation products were analyzed by TLC and high-performance liquid chomatography (HPLC). The results indicated that BaP-7,8-diol was a poor substrate for intestinal sulfotransferase. Vmax for the sulfation of BaP-7,8-diol was 0.002 nmol mg−1 min−1, which is at least three orders of magnitudes lower than the Vmax for phenolic BaP metabolites. Studies with 3′phosphoadenosine-5′phosphosulfate (PAP)-35S as co-substrate showed that an unidentified compound in the reaction mixture was sulfated, dependent on the BaP-7,8-diol concentration. This could indicate that BaP-7,8-diol was interacting with a regulatory site on the enzyme and stimulated sulfation of an endogenous molecule in cytosol. Kinetic analysis of microsomal glucuronidation resulted in a Vmax of 0.30 nmol mg−1 min−1 (±0.06 S.D., n=4), with a Km of 23.39 μM (±2.66 S.D.). The Km for the co-substrate UDP-glucuronic acid was approximately 43 μM. The slow rates for sulfation and glucuronidation of BaP-7,8-diol may explain its relatively high systemic availability when ingested or produced by intestinal phase 1 enzymes.