Xenobiotic Metabolism

Environmental Health Sciences

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Abstract

The majority of our research effort in the past year has been on a class of flame retardants, polybrominated diphenyl ethers (PBDE). PBDE?s are produced commercially as mixtures based on bromine content. For instance, Great Lakes DE-71? (DE-71) is a commercial mixture containing 71% bromine used primarily as a flame retardant in polyurethane foams. DE-71 contains penta-, tetra- , and hexa-brominated diphenyl ethers. PBDE?s are found in mammalian tissues and fluids, including human adipose, serum, and milk. The most prevalent congeners in human samples are BDE-47 (a tetraBDE), BDE-99, and BDE-100 (both pentaBDE?s). These are also the main congeners in the DE-71 commercial mixture. The major hexaBDE is BDE-153 and while it is a minor component compared to the tetra- and penta-congeners, it is highly lipophilic and may be more persistent in vivo. The NTP is currently designing a bioassays for the DE-71 mixture. PBDE?s have relatively low acute toxicity and appear to be non-mutagenic. However, PBDE congeners are structurally similar to TCDD and PCB?s and have been considered to have mechanisms of toxicity in common with those chemicals. Additionally, BDE-47 and its hydroxylated metabolites are structurally similarity to thyroxin and may interfere with thyroid hormone controlled pathways.[unreadable] ADME studies of the congeners, BDE-47, -99, and ?153 have covered a wide range of doses from 0.1 to 1000 umol/kg. 14C-BDE-47, -99 and -153 are well absorbed (70 to 85%) from oral administration of a corn oil formulation. Tissue distribution is linear over the 10,000-fold range of doses, at least for the larger tissues. The metabolism and disposition of 14C-labeled BDE-47 was studied in B6C3f1 mice and F344 rats. Sex and species differences were observed in tissue distribution and excretion of BDE-47 derived radioactivity. Male mice excreted up to 30% of the administered dose in urine unchanged. In general, tissue accumulation was less in mice than rats. Metabolism studies identified gluthathione conjugates in bile. A glucuronide and a sulfate conjugate of 2,4-dibromophenol were identified in urine. This appears to be the first report of metabolic cleavage of a diphenyl ether. The metabolism and disposition of 14C-labeled BDE-99 was studied on F344 rats and B6C3 F1 mice. Within 24 hr following oral doses ranging from 0.1 to 1000 umol/kg to rats, about 50% of the dose was excreted in feces, this includes 16% unabsorbed. Up to 2% was excreted in urine and 34-38% remained in tissues, mostly in fat. Mice excreted more in urine and less in feces than rats. Tissue accumulation was observed following multiple dosing to rats. Two dihydrohydroxy-S-glutathionyl and two S-glutathionyl conjugates of BDE-99, 2,4,5-tribromophenol and its glucuronide, two monoydroxylated BDE-99 glucuronides and three monohyroxylated tetrabromodiphenyl phenyl ether were identified in male rat urine. BDE-99 undergoes more extensive metabolism than BDE-47 or 153. Half of the absorbed oral dose in male rats was excreted in 10 days mostly as metabolites derived from arene oxide metabolism. The disposition of the 14C-labeled polybrominated diphenyl ether (PBDE), 2,2', 4,4',5,5'-hexaBDE (BDE153) was investigated in rodents following single and multiple doses and in a mixture with radiolabeled 2,2',4,4'-tetraBDE (BDE47) and 2,2',4,4',5-pentaBDE (BDE99). In single exposure studies, there was little or no effect of dose on BDE153 disposition in male rats in the range of 1-100 ?mol/kg. No major sex or species differences in the in vivo fate of BDE153 were detected. BDE153 was: 1) approximately 70% absorbed in rats or mice following gavage. 2) retained in tissues. 3) poorly metabolized and slowly excreted. Mixture studies indicated that, relative to each other, more BDE47 was distributed to adipose tissue, more BDE153 accumulated in liver, and BDE99 was metabolized to the greatest extent. BDE153 was probably retained in liver due to minimal metabolism and elimination after "first pass" distribution to the tissue following gavage. Recent NTP studies on the toxicity of pulegone and its furan-containing metabolite menthofuran pointed out the need or more information on the metabolic activation of furan-containing chemicals. In metabolic schemes, it is generally assumed that one of the double bonds in furan is oxidized to an epoxide and the epoxide either reacts or undergoes a rearrangement to a dioxobutene (for example, cis-butenedial from furan). However, formation of the dioxobutene directly would be favored on strictly thermodynamic grounds. The in vitro metabolism of 4-ipomeanol, a human hepatotoxicant, has been investigated in an attempt learn more about metabolic activation of furans. Ipomeanine (IPN), 4-ipomeanol (4-IPO), 1-ipomeanol, and 1,4-ipomeadiol (DIOL) are toxic 3-substituted furans found in mold-damaged sweet potatoes. IPN and 4-IPO are the most toxic, but all produce severe pulmonary toxicity requiring metabolic activation. While the toxicity of these furans is well described, metabolism studies have provided limited data. Initial studies of 4-IPO metabolism by rat liver microsomes demonstrated oxidation of 4-IPO to IPN and reduction to DIOL and that IPN was more easily metabolized to a reactive species than 4-IPO or DIOL. Incubation of IPN and Gly produced a 2?-pyrrolin-5?-one adduct establishing that IPN was metabolized to an enedial metabolite. N-Acetylcysteine reacted with the 5?-aldehyde of the enedial to give two 2?,5?-dihydro-2?-hydroxyfurans stabilized by H-bonding between the 2?-OH and 3?-keto groups. Reaction of the enedial metabolite of IPN with one GSH gave several adducts including a pyrrole derived from 1,2-addition of GSH to the 5?-aldehyde as well as two tricyclic 2?-pyrrolines derived from 1,4-addition of GSH at the 4?-position. The identities of pyrrole and 2?-pyrroline GSH adducts were confirmed by observation of structurally similar adducts from Cys conjugation with the enedial metabolite of IPN. Mono-GSH and bis-GSH adducts were derived from both 1,2-and 1,4-addition of GSH to the enedial metabolite of 4-IPO in rat liver microsomal incubations of 4-IPO and GSH. Sequential oxidation of 4-IPO to IPN then to the enedial metabolite followed by GSH conjugation also occurred in the 4-IPO incubations. None of the observed in vitro metabolites, require the intermediacy of a furan epoxide, while several nitrogen heterocycles can only arise from an amino group reacting with the enedial.