Inhibitory effect of tetrachloro-p-hydroquinone and other metabolites of hexachlorobenzene on hepatic uroporphyrinogen decarboxylase activity with reference to the role of glutathione

Molecular mechanism of metal-independent decomposition of organic hydroperoxides by halogenated quinoid carcinogens and the potential biological implications

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. Organic hydroperoxides (ROOH) can be produced both by free radical reactions and enzymatic oxidation of polyunsaturated fatty acids. ROOH have been shown to decompose to alkoxyl radicals via catalysis by transition metal ions, which may initiate lipid peroxidation or transform further to the reactive aldehydes. However, it is not clear whether XQ react with ROOH in a similar manner to generate alkoxyl radicals metal-independently. By complementary applications of ESR spin-trapping, HPLC/high resolution mass spectrometric and other analytical methods, we found that 2,5-dichloro-1,4-benzoquinone (DCBQ) could significantly enhance the decomposition of a model ROOH tert-butylhydroperoxide, resulting in the formation of t-butoxyl radicals independent of transition metals. On the basis of the above findings, we detected and identified, for the first time, an unprecedented C-centered quinone ketoxy radical. Then, we extended our study to the more physiologically relevant endogenous ROOH 13-hydroperoxy-9,11-octadecadienoic acid and found that DCBQ could also markedly enhance its decomposition to generate the reactive lipid alkyl radicals and the genotoxic 4-hydroxy-2-nonenal (HNE). Similar results were observed with other XQ. In summary, these findings demonstrated that XQ can facilitate ROOH decomposition to produce reactive alkoxyl, quinone ketoxy, lipid alkyl radicals, and genotoxic HNE via a novel metal-independent mechanism, which may explain partly their potential genotoxicity and carcinogenicity.

Effects of hexachlorobenzene on antioxidant status of liver and brain of common carp (Cyprinus carpio)

Hexachlorobenzene (HCB)-induced oxidative damages have been published in rats while the effects have not yet been reported in fishes. Juvenile common carps (Cyprinus carpio) were exposed to waterborne HCB from 2 to 200 microg l-1 for 5, 10 or 20 days. Liver and brain were analyzed for various parameters of oxidative stress. There were no significant changes of glutathione (GSH) content and superoxide dismutase (SOD) activity in liver after 5 or 10 days exposure, whereas obvious drops were observed at higher concentrations after 20 days exposure. Significant decreases of GSH content and SOD activity in brain were found during all the exposure days. In brain, HCB also significantly elevated the contents of reactive oxygen species (ROS), thiobarbituric acid- reactive substances (TBARS, as an indicator of lipid peroxidation products), glutathione disulfide (GSSG), and activities of nitric oxide synthase (NOS), glutathione peroxidase (GPx), and glutathione reductase (GR), and inhibited activities of acetylcholinesterase (AchE) and glutathione S-transferase (GST). The results clearly demonstrated that environmentally possible level of HCB could result in oxidative stress in fish and brain was a sensitive target organ of HCB toxicity.

Polymorphonuclear phagocytosis and killing in workers occupationally exposed to hexachlorobenzene

Phagocytosis and intracellular killing of Candida albicans and Candida pseudotropicalis by neutrophils from 66 workers exposed to chlorinated compounds were studied. Phagocytosis with both antigens was normal in all the workers studied. However, lytic activity of the neutrophils in the presence of both antigens, C. albicans and C. pseudotropicalis was impaired. These findings might be related to a derangement in the antioxidant mechanisms and to some interference with GSH levels that might lead to the abnormal functions observed in neutrophils.

New modes of action of desferrioxamine: scavenging of semiquinone radical and stimulation of hydrolysis of tetrachlorohydroquinone

Desferrioxamine (DFO) is a common drug used in the treatment of iron overload. In addition to its iron-chelation, other properties have been identified. Alas, DFO has demonstrable effects which cannot be explained by its classically established properties; i.e., DFO protects against DNA single strand breaks induced by tetrachlorohydroquinone (TCHQ), while other iron chelators such as DTPA (diethylenetriaminepentaacetic acid) do not. The autooxidation process of TCHQ yielding the tetrachlorosemiquinone radical (TCSQ.) intermediate, was studied here in the presence of chelators. DFO led to a marked reduction in both concentration and life span of TCSQ. via formation of DFO-nitroxide radical (DFO.). In contrast, DTPA had no detectable effect on TCHQ autooxidation. Present studies indicate that the protective effects of DFO on TCHQ-induced DNA damage were not due to the binding of iron, but rather to scavenging of the reactive TCSQ. and the formation of the less reactive DFO.. An additional mode of action of DFO was identified, via stimulation of the hydrolysis (dechlorination) of tetrachloro-1,4-benzoquinone (chloranil), which is the oxidation product of TCHQ, to form 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid). The results of this study demonstrate two new modes of action for DFO: the scavenging of deleterious semiquinone radical, and the stimulation of the hydrolysis of halogenated substituents on the quinone structure. Both modes might prove highly relevant to the biological activities of DFO.

Defective neutrophil function in workers occupationally exposed to hexachlorobenzene

In this work we have studied the respiratory burst and chemotaxis of polymorphonuclear leukocytes from 51 workers exposed to chlorinated compounds, which were compared with those of non-exposed, age- and sex-matched individuals. These two neutrophil functions were significantly reduced as compared to controls. No correlation was observed between the length of exposure, hexachlorobenzene (HCB) blood concentrations and neutrophil chemotaxis or the extent of nitroblue tetrazolium reduction.

2,2',4,4',5,5'-hexabromobiphenyl: its toxicokinetics, biotransformation and porphyrinogenic action in rats

The porphyrinogenic action of 2,2',4,4',5,5'-hexabromobiphenyl and its toxicokinetics were studied in female Wistar rats that were treated every other day for 6 wk with oral doses of 112 mg/kg body weight. Subsequently, the animals were kept for a further period of 22.5 months but without supply of the brominated biphenyl. 10.5 months after cessation of treatment the compound reached a maximum concentration in the adipose tissue followed by a gradual decline of its content. In the liver the concentration of the substance started to decrease 3 months after cessation of treatment. In the excreta, hexabromobiphenylol and two pentabromobiphenyls were detected as metabolites. The rate of biotransformation amounted to about 5%. At the end of the dosing period no alterations in the content and profile of the hepatic porphyrins were observed. Urinary porphyrins and their precursors delta-aminolaevulinic acid and porphobilinogen were slightly elevated. The urinary porphyrin pattern and faecal porphyrin content and pattern did not differ from those of the controls. 15 and 18 months after cessation of treatment (16.5 and 19.5 months after the start of treatment) two animals were found to have marked alterations in the content and profile of hepatic porphyrins with uro- and heptacarboxyporphyrin predominating. It was concluded that there is an extreme delayed individual porphyric response to 2,2',4,4',5,5'-hexabromobiphenyl in female rats.

2,2',3',4,4'5,5'-Hepatachlorobiphenyl (PCB 180)--on its toxicokinetics, biotransformation and porphyrinogenic action in female rats

The toxicokinetics and biotransformation of 2,2',3',4,4',5,5'-heptachlorobiphenyl, as well as its influence on the activity of microsomal and cytosolic enzymes and on the porphyrin pathway in the liver were studied in female rats following oral treatment with 7 mg/kg every other day for 3 months. One day after cessation of treatment the concentration of the compound in liver, spleen, CNS and blood was 100-500 times and in the trachea it was only 5 times less than in the adipose tissue. The daily excretion with the feces and urine amounted to 35 and 1.5 micrograms, respectively. In both excreta, heptachlorobiphenylol was identified as a metabolite. The biotransformation rate was estimated to be about 5%. Investigations of the liver revealed increases in the relative liver weight, total cytochrome P-450 content, O-deethylation of 7-ethoxycoumarin and in the activity of glutathione S-transferases. Disturbances of the hepatic porphyrin pathway were not detected. Only at the end of a post-dosing period of 12 months did the hepatic uroporphyrinogen decarboxylase show diminished activity. Only one of these animals with diminished enzyme activity showed drastically elevated porphyrins. In these animals, the fecal and urinary porphyrins did not differ from controls. At no time did heptachlorobiphenyl influence the urinary excretion of delta-aminolevulinic acid and porphobilinogen. The results indicate 1) that this congener shows expected toxicokinetics with the exception of being accumulated in the trachea and 2) that this congener induces disturbances of the hepatic porphyrin pathway several months after cessation of treatment.

Sex-related difference in hepatic glutathione conjugation of hexachlorobenzene in the rat

Hexachlorobenzene (HCB) induces hepatic porphyria and liver cancer in female rats, whereas toxicity is minimal in male rats. HCB is biotransformed to sulfur-containing metabolites originating from conjugation to glutathione (GSH). This study aimed to assess differences in GSH conjugation of HCB between male and female rats. Sprague-Dawley rats of both sexes were given (po, 10 ml/kg in corn oil) five consecutive doses of 100 mg/kg HCB [2 bid (7:30, 15:30) + 1 sid (7:30)]. This cumulative dose produced porphyria in female but not male rats after a delay period of 6 weeks. Animals were killed 0, 6, 12, 18, or 24 hr after the last dose. Hepatic GSH level showed a diurnal cycle in rats of both sexes, but it was more pronounced in males; the minimum level was observed at 12 hr after dosing. The GSH level in HCB-treated male rats was significantly lower than control at 6, 18, and 24 hr, whereas no significant differences were observed for HCB-treated female rats. Biliary excretion of pentachlorothiophenol, a metabolite originating from GSH conjugation of HCB, was higher in male than female rats. Liver cytosolic GSH transferase activity toward 3,4-dichloronitrobenzene was significantly higher than control level in male but not female rats given HCB. GSH transferase activity toward 1,2-epoxy-3-(p-nitrophenoxy)propane in male and female rats was not increased by HCB treatment. The liver HCB concentration at 24 hr after dosing was higher in male rats than in female rats but decreased faster thereafter. These results suggest that hepatic GSH conjugation of HCB is more important in male than in female rats. This may be related to the reduced liver porphyria observed in HCB-treated male rats compared to female rats.