Sulfonation of 17beta-estradiol and inhibition of sulfotransferase activity by polychlorobiphenylols and celecoxib in channel catfish, Ictalurus punctatus

Abstract

The sulfonation of 17beta-estradiol (E2) by human liver and recombinant sulfotransferases is influenced by environmental contaminants such as hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs), which are potent inhibitors, and the therapeutic drug, celecoxib, which affects positional sulfonation of E2. In some locations, the aquatic environment is contaminated by PCBs, OH-PCBs and widely used therapeutic drugs. The objectives of this study were to investigate the sulfonation kinetics of E2 in liver cytosol from channel catfish (Ictalurus punctatus); to examine the effect of OH-PCBs on E2 sulfonation; and to determine if celecoxib altered the position of E2 sulfonation, as it does with human liver cytosol. E2 was converted to both 3- and 17-sulfates by catfish liver cytosol. At E2 concentrations below 1 microM, formation of E2-3-sulfate (E2-3-S) predominated, but substrate inhibition was observed at higher concentrations. Rates of E2-3-S formation at different E2 concentrations were fit to a substrate inhibition model, with K'm and V'max values of 0.40 +/- 0.10 microM and 91.0 +/- 4.7 pmol/min/mg protein, respectively and K(i) of 1.08 +/- 0.09 microM. The formation of E2-17-S fit Michaelis-Menten kinetics over the concentration range 25 nM to 2.5 microM, with K(m) and V(max) values of 1.07 +/- 0.23 microM and 25.7 +/- 4.43 pmol/min/mg protein, respectively. The efficiency (V(max)/K(m)) of formation of E2-3-S was 9.8-fold higher than that of E2-17-S. Several OH-PCBs inhibited E2 3-sulfonation, measured at an E2 concentration of 1 nM. Of those tested, the most potent inhibitor was 4'-OH-CB79, with two chlorine atoms flanking the OH group (IC(50): 94 nM). The inhibition of estrogen sulfonation by OH-PCBs may disrupt the endocrine system and thus contribute to the known toxic effects of these compounds. Celecoxib did not stimulate E2-17-S formation, as is the case with human liver cytosol, but did inhibit the formation of E2-3-S (IC(50): 44 microM) and to a lesser extent, E2-17-S (IC(50): > 160 microM), suggesting the previously found effect of celecoxib on E2-17-S formation may be specific to human SULT2A1.

Fig. 1

Structure of celecoxib and OH-PCBs used in this study

Fig. 2

Results for rates of E2 sulfonation in catfish liver cytosol. Substrate inhibition of the formation of E2-3-S is shown in (A). The formation of E2-17-S is shown in (B). Data shown are the mean values from studies with three catfish, and error bars indicate standard deviation. The kinetic parameters are summarized in Table 1.

Fig. 3

Inhibition of E2 sulfonation with catfish liver cytosol by OH-PCBs. The sulfotransferase activity is given as percentage of control activity. Data given are the mean ± S.D. of experiments with three fish.

Fig. 4

Inhibition of E2 sulfonation with catfish liver cytosol by celecoxib. The sulfotransferase activity is given as percentage of control activity with 0.8 μM E2. Data given are the mean ± S.D. of experiments with three fish.