Characterizing the in vitro hepatic biotransformation of the flame retardant BDE 99 by common carp

Polybrominated diphenyl ethers (PBDEs) are a class of flame retardant chemicals known to biomagnify in aquatic foodwebs. However, significant biotransformation of some congeners via reductive dehalogenation has been observed during in vivo and in vitro laboratory exposures, particularly in fish models. Little information is available on the enzyme systems responsible for catalyzing this metabolic pathway in fish. This study was undertaken to characterize the biotransformation of one primary BDE congener, 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), using in vitro techniques. Hepatic sub-cellular fractions were first prepared from individual adult common carp (Cyprinus carpio) to examine metabolism in both microsomal and cytosolic sub-cellular fractions. Debromination rates (i.e. BDE-99 biotransformation to BDE-47) were generally higher in the microsomal fraction than in the cytosolic fraction, and some intra-species variability was observed. Further experiments were conducted to determine the biotransformation kinetics and the influence of specific co-factors, inhibitors and competitive substrates on metabolism using pooled carp liver microsomes. The apparent K(m) and V(max) values were 19.4microM and 1120pmolesh(-1)mgprotein(-1), respectively. Iodoacetate (IaC) and the two thyroid hormones, reverse triodothyronine (rT3) and thyroxine (T4), significantly inhibited the debromination of BDE-99 in microsomal sub-cellular fractions with IC(50) values of 2.2microM, 0.83microM, and >1.0microM, respectively. These results support our hypothesis that deiodinase enzymes may be catalyzing the metabolism of PBDEs in fish liver tissues. Further studies are needed to evaluate metabolic activity in other species and tissues that contain these enzymes.

Characterization of an iodothyronine 5'-deiodinase in gilthead seabream (Sparus auratus) that is inhibited by dithiothreitol

Iodothyronine deiodinases catalyze the conversion of the thyroid prohormone T(4) to T(3) by outer ring deiodination (ORD) of the iodothyronine molecule. The catalytic cycle of deiodinases is considered to be critically dependent on a reducing thiol cosubstrate that regenerates the selenoenzyme to its native state. The endogenous cosubstrate has still not been firmly identified; in studies in vitro the sulfhydryl reagent dithiothreitol (DTT) is commonly used to activate ORD. We now have characterized an ORD activity in the teleost gilthead seabream (Sparus auratus) that is inhibited by DTT. DTT inhibited reverse T(3) (rT(3)) ORD by 70 and 100% in kidney homogenates (IC(50) 0.4 mmol/liter) and microsomes (IC(50) 0.1 mmol/liter), respectively. The omission of DTT from the incubation medium restored renal ORD Michaelis-Menten kinetics with a Michaelis constant value of 5 mumol/liter rT(3) and unmasked the inhibition by 6-n-propyl-2-thiouracil. A putative seabream deiodinase type 1 (saD1), derived from kidney mRNA, showed high homology (> or = 41% amino acid identity) with vertebrate deiodinases type 1. Features of this putative saD1 include a selenocysteine encoded by an in-frame UGA codon, consensus sequences, and a predicted secondary structure for a selenocysteine insertion sequence and an amino acid composition of the catalytic center that is identical with reported consensus sequences for deiodinase type 1. Remarkably, three of six cysteines that are present in the deduced saD1 protein occur in the predicted amino terminal hydrophobic region. We suggest that the effects of DTT on rT(3) ORD can be explained by interactions with the cysteines unique to the putative saD1 protein.

Triiodothyronine (T3) antagonizes adverse effects of high circulating reverse-T3 (rT3) during hemorrhagic shock

To examine whether triiodothyronine (T3) could counteract the lethal effect of exogenous reverse T3 (rT3) in hemorrhagic shock, 21 anesthetized, heparinized mongrel dogs were given 15 micrograms/kg of rT3 IV. Thirty minutes later, the dogs were bled rapidly into a reservoir to achieve and maintain a mean arterial pressure of 40 mm Hg. After 60 minutes at 40 mm Hg (compensated shock), the reservoir line was clamped for 30 minutes (uncompensated shock). The shed blood was then reinfused over 30 minutes, and the dogs were monitored for an additional 60 minutes. At the start of uncompensated shock, 11 dogs were given at least 15 micrograms/kg of T3 IV, and 10 animals received saline. Before T3 treatment, there were no significant intergroup differences in the measured hemodynamic and blood gas variables. In the untreated group, 8 of 10 dogs (80%) died during uncompensated shock, in comparison to 3 of 11 dogs (27%) that received T3 (P less than 0.01). Long-term survival in the T3 group was 5/11 (45%), significantly higher than that (1/10, 10%) in the untreated group (P less than 0.05). These results, interpreted in relationship to previous studies, suggest that the therapeutic efficacy of T3 in canine hemorrhagic shock may be related to antagonism of adverse effects of endogenous rT3.