Physicochemical properties of hydroxylated polychlorinated biphenyls aid in predicting their interactions with rat sulfotransferase 1A1 (rSULT1A1)

Abstract

Hydroxylated metabolites of polychlorinated biphenyls (OHPCBs) interact with rat sulfotransferase 1A1 (rSULT1A1) as substrates and inhibitors. Previous studies have shown that there are complex and incompletely understood structure-activity relationships governing the interaction of rSULT1A1 with these molecules. Furthermore, modification of the enzyme with glutathione disulfide (GSSG) results in the conversion of some OHPCBs from inhibitors to substrates. We have now examined estimated values for the acid-dissociation constant (Ka) and the octanol-water distribution coefficient (D), as well as experimentally determined dissociation constants for enzyme complexes, to assist in the prediction of interactions of OHPCBs with rSULT1A1. Under reducing conditions, initial velocities for rSULT1A1-catalyzed sulfation exhibited a positive correlation with pKa and a negative correlation with log D of the OHPCBs. IC50 values of inhibitory OHPCBs decreased with decreasing pKa values for both the glutathione (GSH)-pretreated and GSSG-pretreated forms of rSULT1A1. Comparison of GSH- and GSSG-pretreated forms of rSULT1A1 with respect to binding of OHPCB in the presence and absence of adenosine 3’,5’-diphosphate (PAP) revealed that the dissociation constants with the two redox states of the enzyme were similar for each OHPCB. Thus, pKa and log D values are useful in predicting the binding of OHPCBs to the two redox forms of rSULT1A1 as well as the rates of sulfation of those OHPCBs that are substrates. However, the differences in substrate specificity for OHPCBs that are seen with changes in redox status of the enzyme are not directly related to specific structural effects of individual OHPCBs within inhibitory enzyme-PAP-OHPCB complexes.

Modification of the Catalytic Function of Human Hydroxysteroid Sulfotransferase hSULT2A1 by Formation of Disulfide Bonds

“…ANS was used as a fluorescent probe for determination of the binding of ligands (e.g., DHEA and PAP) to both the unmodified and the oxidized hSULT2A1 using the previously described procedure for the study of rSULT1A1 (Marshall et al, 1997;Liu et al, 2011) with a modification wherein concentrations of ANS were selected by determining conditions of saturation under the conditions that were used in the assay (Supplemental Fig. 1).…”

“…Although the various isoforms often have distinct, but overlapping, specificities for substrates and inhibitors, it is becoming increasingly apparent that other factors, such as the redox environment of these enzymes, can have additional effects on catalysis. For example, several family 1 SULTs are sensitive to oxidants that cause the formation of disulfide bonds (Marshall et al, 1997(Marshall et al, , 2000Duffel et al, 2001;Maiti et al, 2005Maiti et al, , 2007Liu et al, 2011;Dammanahalli and Duffel, 2012). Moreover, it is clear that, in addition to the rates of catalysis, substrate specificity may be altered by disulfide bond formation in family 1 SULTs (Marshall et al, 2000;Duffel et al, 2001;Liu et al, 2011).…”

Metabolism and metabolites of polychlorinated biphenyls

“…While many OH-PCB metabolites are good substrates for conjugation reactions (James, 2001), the ease of formation of glucuronic acid and sulfate conjugates is highly structure-dependent, an observation that is supported by the persistence of certain OH-PCBs in serum (Tampal et al, 2002, Liu et al, 2009, Bergman et al, 1994b, Gutleb et al, 2010). In addition, both conjugation reactions, glucuronidation and sulfation, may be inhibited by OH-PCBs (Ekuase et al, 2011, James, 2001, Kester et al, 2000, Liu et al, 2006, Liu et al, 2011, Liu et al, 2009, Schuur et al, 1998b, van den Hurk et al, 2002, Wang et al, 2006). …”

“…Although the toxicological relevance of PCB sulfation is not yet known, a variety of in vitro and in vivo studies suggest that the formation of PCB sulfates is a potentially significant metabolic pathway for LC-PCBs in humans (Dhakal et al, 2012, Dhakal et al, 2014, Ekuase et al, 2011, Grimm et al, 2013, Liu et al, 2006, Liu et al, 2011, Liu et al, 2009, Zhai et al, 2013a). The 36 fact that, in the past, human serum has been almost exclusively analyzed for parent PCBs and only recently for their hydroxylated metabolites indicates that overall PCB exposure levels in exposed populations may have been underestimated, at least with respect to the LC-PCBs (Hovander et al, 2000, Marek et al, 2013b, Dirtu and Covaci, 2010).…”

Identification of a sulfate metabolite of PCB 11 in human serum

“…Recent in vitro and in vivo studies revealed SULT-catalyzed sulfation of OH-PCBs as a major reaction in the metabolism of LC-PCBs in the rat (Dhakal et al. 2012; Ekuase et al 2011; Liu et al 2011). In vivo concentrations of sulfate conjugates were significantly higher than those of glutathione conjugate derivatives and glucuronide metabolites in serum of rats exposed to PCB 3 (Dhakal et al 2012).…”