Structure-activity relationships for interactions of hydroxylated polychlorinated biphenyls with human hydroxysteroid sulfotransferase hSULT2A1

Abstract

Industrial chemicals known as polychlorinated biphenyls (PCBs) were widely used for decades until their production was banned worldwide due to their persistence and toxicities to humans and other animals. Upon oxidative metabolism by cytochrome P450, hydroxylated metabolites of PCBs (OHPCBs) are formed. OHPCBs have been shown to competitively displace thyroxine from transthyretin, block normal hormonal activity, and inhibit phenol or family 1 sulfotransferases (SULTs) which catalyze sulfation of thyroid hormones and estrogens. Recently, three OHPCBs were shown to also interact with hydroxysteroid or family 2 sulfotransferases that play a role in the homeostasis of steroid hormones such as dehydroepiandrosterone (DHEA). The objectives of the studies presented in this dissertation were to further examine the effects of selected OHPCBs on the activity of human hydroxysteroid sulfotransferase (hSULT2A1), to develop a three-dimensional quantitative structure activity relationship (3D-QSAR) model for OHPCBs as inhibitors of DHEA-sulfation catalyzed by this enzyme, and to investigate the mechanism of inhibition and binding of OHPCBs to hSULT2A1. All 15 OHPCBs examined inhibited the sulfation of 1 µM [ 3 H] DHEA, catalyzed by hSULT2A1 with IC 50 values ranging from 0.6 to 96 µM. The OHPCBs with a 3, 5dichloro-4-hydroxy substitution were the most potent inhibitors of DHEA sulfation, and they were also shown to be substrates for hSULT2A1. Eight OHPCBs were substrates for hSULT2A1, and seven were solely inhibitors (i.e. they inhibited the sulfation of DHEA, yet they were not themselves sulfuryl-acceptors in hSULT2A1-catalyzed reactions). A 3D-QSAR model was developed utilizing comparative molecular field analysis (CoMFA). The model fit the data well and also had good predictability. The kinetics of inhibition showed that these OHPCBs were noncompetitive inhibitors of hSULT2A1. Binding studies utilizing the displacement of a fluorescent iii ACKNOWLEDGMENTS I would like to express my immense gratitude to my advisor, Dr. Duffel. Thank you for your patience, mentorship and support over the years. I could not have achieved this milestone without your help. You were more than a mentor to me; you were like a friend. I came to you with all my problems, be it research related or personal, and you always gave me valuable advice on what to do. I say thank you, thank you and thank you again.

3,3′-Dichlorobiphenyl Is Metabolized to a Complex Mixture of Oxidative Metabolites, Including Novel Methoxylated Metabolites, by HepG2 Cells

“…Specifically, the formation of 4-OH-PCB 11 and the corresponding sulfate is consistent with a human biomonitoring study 18,19 and an in vitro study demonstrating that 4-OH-PCB 11 is a substrate for human sulfotransferases. 25 Other PCB congeners appear to be preferentially metabolized to para-hydroxylated and para/meta-dihydroxylated metabolites by human cytochrome P450 enzymes. 70,89 However, additional metabolism studies are needed with experimental models that better approximate the hepatic metabolism and clearance in humans than HepG2 cells, such as primary hepatocytes, HepaRG cells, hepatospheres, and others.…”

“…23,24 4-OH-PCB 11 is a substrate of human SULT2A1 and inhibits the sulfation of steroids, such as dehydroepiandrosterone (DHEA), by sulfotransferases (SULTs). [25][26][27] Moreover, 4-OH-PCB 11 is likely a substrate for UDPglucuronosyltransferases (UGTs) and, as with other hydroxylated PCBs, 28,29 likely metabolized to a glucuronide in vivo. As suggested from observations with polar bears, 30 hydroxylated, sulfated and glucuronidated PCB 11 metabolites may be further oxidized to diverse di-and tri-hydroxylated metabolites.…”