Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the in vivo and in vitro dechlorination of pentachlorophenol

Metabolic studies on pentachlorophenol (PCP) in rats

1. The metabolism of pentachlorophenol in rats was studied. 2. Metabolites isolated from rat urine and identified were: 2,3,4,5-tetrachlorophenol, 2,3,4,6-tetrachlorophenol, 2,3,5,6-tetrachlorophenol, tetrachlorocatechol, tetrachlororesorcinol, trichlorohydroquinone, tetrachlorohydroquinone, and traces of trichloro-1,4-benzoquinone, and tetrachloro-1,4-benzoquinone.

The in vitro metabolites of 2,4,6-trichlorophenol and their DNA strand breaking properties

The carcinogenic compound 2,4,6-trichlorophenol (2,4,6-TCP) was incubated with rat liver S-9 fraction. Three metabolites were identified: 2,6-dichloro-1,4-hydroquinone (DHQ), and two isomers of hydroxypentachlorodiphenyl ether (OH-Cl5-DPE). The latter are probably products of microsomal .OH radical attack on the trichlorophenol molecule forming phenoxy free radicals. These would undergo dimerizations with other molecules present in solution. The 2,6-dichloro-1,4-semiquinone free radical was identified by ESR spectroscopy. It is formed at physiological conditions in phosphate buffer at pH 7.2 and 7.8, with a more intensive signal at the more alkaline pH. The formation is probably due to the autoxidation of the corresponding hydroquinone. Incubation of a mixture of metabolites with PM2 DNA at pH 7.2 resulted in single strand breaks. Addition of catalase and dimethylsulfoxide (DMSO) inhibited the DNA strand scission. It was concluded that reactive oxygen species (ROS), produced during the formation of the semiquinone radical, were responsible for the observed DNA damage. The significance of the ROS and the semiquinone free radical is discussed in view of the reported tumorgenicity of 2,4,6-TCP in rats and mice.

The microsomal metabolism of pentachlorophenol and its covalent binding to protein and DNA

The microsomal metabolism of pentachlorophenol (PCP) was investigated, with special attention to the conversion dependent covalent binding to protein and DNA. The two metabolites detected were tetrachloro-1,2- and tetrachloro-1,4-hydroquinone. Microsomes from isosafrole (ISF)-induced rats were by far the most effective in catalyzing the reaction: the rate of conversion was increased 7-fold over control microsomes. All other inducers tested (hexachlorobenzene (HCB), phenobarbital (PB) and 3-methylcholanthrene (3MC) gave 2--3-fold increases over control. There are indications that the 1,2- and 1,4-isomers are produced in different ratio's by various cytochrome P-450 isoenzymes: Microsomes from PB- and HCB-treated rats produced the tetrachloro-1,4- and tetrachloro-1,2-hydroquinone in a ratio of about 2, while microsomes from rats induced with 3 MC and ISF showed a ratio of about 1.3. When PCP was incubated with microsomes from rats treated with HCB, a mixed type inducer of P-450, the ratio between formation of the 1,4- and 1,2-isomers decreased with increasing concentration of PCP, suggesting the involvement of at least two P-450 isoenzymes with different Km-values. The overall apparent Km-value for HCB-microsomes was 13 microM both for the formation of the soluble metabolites and the covalent binding to microsomal protein, suggesting both stem from the same reaction. The covalent binding could be inhibited by ascorbic acid and this inhibition was accompanied by an increase in formation of tetrachlorohydroquinones (TCHQ). Although a large variation was observed in rates of conversion between microsomes treated with different (or no) inducers, the rate of covalent binding to microsomal protein was remarkably constant. A conversion-dependent covalent binding to DNA was observed in incubations with added DNA which was 0.2 times the amount of binding to protein (37 pmol/mg DNA).

Metabolism of the "mixed" cytochrome P-450 inducer hexachlorobenzene by rat liver microsomes

Hexachlorobenzene (HCB) was metabolised by phenobarbital-induced liver microsomes from male rats to pentachlorobenzene, pentachlorophenol, tetrachloro-1,2-benzenediol and tetrachloro-1,4-benzenediol (1:88:2:9). Metabolites were identified and quantified by electron capture g.l.c. Structures were confirmed by selective ion monitoring g.l.c.-m.s. The formation of pentachlorophenol was dependent on the presence of NADPH and O2 and inhibited by CO, SKF 525A and metyrapone. Conversion of HCB to pentachlorophenol was stimulated by pretreatment of rats with phenobarbital (PB) but not by 3-methylcholanthrene (3-MC), or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In contrast, the conversion of pentachlorophenol to tetrachloro-1,4-benzenediol was markedly induced by 3-MC but poorly by PB. HCB, Aroclor 1254 and isosafrole stimulated both hydroxylations. The cytochrome P-450c inhibitor 9-hydroxyellipticine inhibited conversion of pentachlorophenol to tetrachlorobenzenediols by HCB and beta-naphthoflavone induced micromes. In addition to hydroxylation reactions, evidence was obtained for the conjugation of HCB with glutathione catalysed by a microsomal glutathione transferase. Radioactivity from [14C]HCB was bound to microsomal protein during aerobic incubations. Binding was inhibited by GSH and N-acetyl-cysteine. Preliminary studies suggested that the reactive species was derived from tetrachloro-1,4-benzoquinone. No correlation was found between levels of metabolites or covalent binding produced by the two sexes and the marked sex dependent hepatic porphyrogenic and carcinogenic effects of HCB.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on the hepatic storage of retinol in rats with different dietary supplies of vitamin A (retinol)

The effect of various dietary sources of vitamin A on liver storage of retinol has been investigated in Sprague-Dawley rats treated with single oral doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): 0,0.1,1.0, or 10 microgram.kg-1. Each dose group consisted of 3 subgroups, each comprising 10 rats which received a diet with normal, low or high retinol content. The animals were killed 4 weeks after TCDD administration. Analyses of retinol were performed by high pressure liquid chromatography and glucuronosyltransferase activities were determined spectrophotometrically. A dose-dependent decrease in hepatic storage of retinol was evident. The high retinol diet did not fully compensate for the reduction caused by the highest TCDD-dose. Glucuronosyltransferase activity increased directly in relation to the TCDD-dose but in inverse proportion to the retinol content of the diet.

Chlorinated phenols: occurrence, toxicity, metabolism, and environmental impact

Pentachlorophenol and the lower chlorinated phenols, tetra- and trichlorophenols, have gained an increasing use as fungicides, herbicides, insecticides, and precursors in the synthesis of other pesticides since the early 1930s. World-wide production totals about 200,000 tons. Production and use of chlorinated phenols have caused industrial hygiene problems but, otherwise, have not been recognized to create more than limited environmental problems. The introduction of modern analytical techniques, however, has revealed the ubiquitous occurrence of chlorophenols in the environment, and the discovery of chlorinated dimers, such as dibenzo-p-dioxins and dibenzofurans, as impurities in commercial chlorophenol formulations, has made a reevaluation of the chlorinated phenols necessary. The present article reviews recent studies on the toxicity and metabolism in mammals and aquatic organisms and the degradation of the chlorophenols under various conditions in the environment. Finally, the hazards of burning of chlorophenol wastes are discussed, as well as health considerations with regard to humans and the environment.

Vitamin A (retinol) status in the rat after a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effect of a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 10 microgram . kg-1, on the status of vitamin A (retinol) was studied in rats during an eight week period after administration. Retinol was determined spectrophotometrically after separation by means of high pressure liquid chromatography. In the liver of control animals the total storage and the concentration of retinol were found to increase linearly with time whilst in the TCDD-exposed animals both the concentration and total storage remained essentially unchanged. Differences in the storage levels were evident after 4 days, and after 8 weeks the treated animals had a total liver storage corresponding to about 30% of the controls. Retinol levels in serum were significantly higher in the treated animals after week 1 and 2.

Metabolism of tetrachlorophenols in the rat

The three isomers of tetrachlorophenol were administrated intraperitoneally to rats and the urinary excretion products studied. Tetrachloro-p-hydroquinone was identified as a major metabolite of 2,3,5,6-tetrachlorophenol, constituting about 35% of the dose given. Trichloro-p-hydroquinone was identified as a minor metabolite of both 2,3,4,5- and 2,3,4,6-tetrachlorophenol. 2,3,5,6-tetrachlorophenol was eliminated within 24 h, 2,3,4,6-tetrachlorophenol within 48 h while only 60% of the given dose of 2,3,4,5-tetrachlorophenol could be recovered within 72 h. The acute toxicity of the tetrachlorophenols and tetrachloro-p-hydroquinone was studied in mice upon oral and intraperitoneal administration. 2,3,5,6-tetrachlorophenol (LD50p.o. 109 mg . kg-1) was the most toxic compound followed by 2,3,4,6- and 2,3.4,5-tetrachlorophenol (LD50p.o. 131 and 400 mg . kg-1, respectively). Tetrachloro-p-hydroquinone proved to have low oral toxicity (LD50p.o. 500 mg . kg-1) but was more toxic than the tetrachlorophenols when administered intraperitoneally. The oral LD50 for pentachlorophenol, under identical experimental conditions was found to be 74 mg . kg-1.