Differences between Human and Rat Intestinal and Hepatic Bisphenol A Glucuronidation and the Influence of Alamethicin on In Vitro Kinetic Measurements

Abstract

ABSTRACT:The extent to which membrane-disrupting agents, such as alamethicin, may alter cofactor transport and influence in vitro kinetic measurements of glucuronidation is a major concern regarding the characterization and extrapolation of inter-and intraspecies pharmacokinetics of bisphenol A (BPA). An additional concern is the omission of a BPA intestinal metabolism component in current pharmacokinetic models used to assess oral exposure. In this study, BPA glucuronidation in native hepatic microsomes from female rat and female human liver displayed higher V max values than that in males. In the presence of alamethicin, all hepatic V max values increased; however, this increase was disproportionately greater in males and gender differences were no longer observed. Female rats exhibited a much higher K m than all other species and genders; the addition of alamethicin had little influence on K m values for any of the test systems. The dissimilar K m measured for female rat suggests that different UDP-glucuronosyltransferase (UGT) enzyme(s) are involved in BPA glucuronidation. The presence of different UGTs in female rat was confirmed using Hill coefficients measured from diclofenac-mediated chemical inhibition assays within hepatic microsomes and purified human UGT2B7 and UGT2B15. Mixed-gender human intestinal microsomes showed little BPA glucuronidation reactivity compared with those from male rat intestine. Male rat intestinal microsomes in the presence of alamethicin exhibited a V max that was nearly 30-fold higher than that for mixed human microsomes. The species and gender metabolic differences we observed between rat and human liver and intestine provide key information for delineating BPA pharmacokinetics needed for human health risk assessment.

Conjugation and Deconjugation Reactions within the Fetoplacental Compartment in a Sheep Model: A Key Factor Determining Bisphenol A Fetal Exposure

“…This suggests that UGT's activities toward BPA are similar in male and female fetal livers at the examined stages of pregnancy. This result is in agreement with data obtained with human liver microsomes (Elsby et al, 2001;Mazur et al, 2010) and hepatocytes (Kuester and Sipes, 2007), as well as with the BPA pharmacokinetic data previously reported for sheep (Corbel et al, 2013). By contrast, a gender difference in the hepatic glucuronidation of BPA was observed in vitro in adult rats (Mazur et al, 2010).…”

“…The estimated value of hepatic glucuronidation clearance for adult humans (7.7 ml/kg per min) was consistent with the values determined in in vitro studies using liver microsomal fractions [13.7 ml/kg per min (Elsby et al, 2001); 15.5 ml/kg per min (Mazur et al, 2010)] or cryopreserved hepatocytes [4.4 ml/kg per min (Kurebayashi et al, 2010); 6 ml/kg per min (Kuester and Sipes, 2007)]. …”

“…In human liver microsomes, BPA is mainly glucuronidated by the UGT2B15 and 2B7 isoforms (Hanioka et al, 2008;Mazur et al, 2010). The expression of these two isoforms is detectable in human fetal livers during the second trimester of pregnancy, and has been stated to account for 18% (Divakaran et al, 2014) or 10%-20% of the values calculated in adults.…”

“…where C was the microsomal protein concentration in the incubation medium (mg/ml) and logP/D, corresponding to logKow for a basic compound, was 3.4 for BPA (Austin et al, 2002;Mazur et al, 2010). To verify the predicted fu inc determined by eq.…”

A roadmap for the development of alternative (non-animal) methods for systemic toxicity testing

“…The significance of intestinal phase II metabolism to total chemical clearance is another factor to be considered in IVIVE of metabolism. Intestinal glucuronidation of BPA demonstrates the importance of consideration of intestinal metabolism to provide key information for describing BPA biokinetics for human health risk assessment based on in vitro metabolism information (Mazur et al, 2010). Compared to IVIVE of hepatic metabolism data, there are several challenges in extrapolating in vitro intestinal metabolism parameters to in vivo.…”