The Utilization of Cellular and Subcellular Liver Models to Assess Glucronidation of Bisphenol A in Animals and Humans

Abstract

Bisphenol A (BPA) is a phenolic compound with industrial and commercial uses. In the liver BPA undergoes extensive metabolism and is eliminated primarily as a glucuronide conjugate. This dissertation compared the metabolism BPA in hepatocytes (HC) to those observed with hepatic microsomes (mics). These values were then scaled to predict the total hepatic metabolic clearance (CLmet ) and compared to human hepatic blood flow (HBF, 20 ml/min/kg).The mean CLint for human HC was 126±63 and 149±72 ml/min/kg for male female HC, respectively. When CLint was scaled to account for HBF, the estimated CLmet values were 16.9 and 17.3 ml/min/kg for male and female HC, respectively.To understand how age, gender, strain and species affects BPA glucuronidation, CLint of BPA was determined from incubations of mics isolated from adult male and female Sprague Dawley (SD) rats (77-day old), newborn (21- day), fetal (gestational day 19 [GD-19]) SD rats. Additional incubations were conducted with mics from adult F-344 rats, CF1 mice and humans. All specimens (HC and mics) were incubated with various concentrations of [14C]-BPA and the formation of a BPA-glucuronide (predominant metabolite) followed Michaelis Menton kinetics.Mics from adult F-344 rats formed BPA glucuronide similar those obtained from SD rats. Fetal mics showed a decreased capacity to glucuronidate BPA as compared to mics of adult rats. These data demonstrate that the glucuronsyltransferase activity responsible for BPA metabolism is developed before GD-19 and increases soon after birth. The rates of BPA-glucuronide formation (in vitro) were observed to be related to body mass with the mouse having the largest CLint and humans having the lowest.The differences in glucuronidation rates observed in vitro were minimized when values were scaled to account for the total metabolic capacity of the liver. These estimated CLmet values represent 85% to 90% of HBF which explained the very low levels of free BPA observed in the blood of human volunteers who ingested BPA (5 mg/person). Thus, hepatic glucuronidation is an efficient "first pass" mechanism which greatly reduces systemic exposure to BPA ingested orally.