EXPERIMENTAL PORPHYRIA IN RATS INDUCED BY CHLORINATED BENZENES

A comparative human health risk assessment of p-dichlorobenzene-based toilet rimblock products versus fragrance/surfactant-based alternatives

A comparison of the human health risk to consumers using one of two types of toilet rimblock products, either a p-dichlorobenzene-based rimblock or two newer fragrance/surfactant-based alternatives, was conducted. Rimblock products are designed for global use by consumers worldwide and function by releasing volatile compounds into indoor air with subsequent exposure presumed to be mainly by inhalation of indoor air. Using the THERdbASE exposure model and experimentally determined emission data, indoor air concentrations and daily intake values were determined for both types of rimblock products. Modeled exposure concentrations from a representative p-dichlorobenzene rimblock product are an order of magnitude higher than those from the alternative rimblock products due to its nearly pure composition and high sublimation rate. Lifetime exposure to p-dichlorobenzene or the subset of fragrance components with available RfD values is not expected to lead to non-cancer-based adverse health effects based on the exposure concentrations estimated using the THERdbASE model. A similar comparison of cancer-based effects was not possible as insufficient data were available for the fragrance components.

Experimental hepatic uroporphyria induced by the diphenyl-ether herbicide fomesafen in male DBA/2 mice

Hepatic uroporphyria can be readily induced by a variety of treatments in mice of the C57BL strains, whereas DBA/2 mice are almost completely resistant. However, feeding of the protoporphyrinogen oxidase-inhibiting herbicide fomesafen (0.25% in the diet for 18 weeks) induced hepatic uroporphyria in male DBA/2N mice (liver porphyrin content up to 150 nmol/g, control animals 1 nmol/g), whereas fomesafen-treated male C57BL/6N mice displayed only a slight elevation of liver porphyrins (approximately 5 nmol/g). The profile of accumulated hepatic porphyrins in fomesafen-treated DBA/2N mice resembled the well-characterised uroporphyria induced by polyhalogenated aromatic hydrocarbons, while histological examination confirmed the presence of uroporphyria-specific cytoplasmic inclusions in the hepatocytes. Uroporphyrinogen decarboxylase activity decreased to about 30% of control values in fomesafen-treated DBA/2N mice; microsomal methoxyresorufin O-dealkylase activity was slightly reduced. The amount of CYP1A1 and CYP1A2 mRNA, as determined by real-time PCR, was not significantly changed; mRNA encoding the housekeeping 5-aminolevulinic acid synthase was elevated 10-fold. Total liver iron was slightly increased. A similar uroporphyria was induced by the herbicide formulation Blazer, containing a structurally related herbicide acifluorfen, when fed to DBA/2N mice at a dose corresponding to 0.25% of acifluorfen in the diet. Since DBA/2 mice are almost completely resistant to all well-characterised porphyrogenic chemicals, the results suggest the possible existence of a yet unknown mechanism of uroporphyria induction, to which the DBA/2 mouse strain is more sensitive than the C57BL strain.

The contribution of 2,3,5-trichlorophenyl methyl sulfone, a metabolite of 1,2,4-trichlorobenzene, to the delta-aminolevulinic acid synthetase induction by 1,2,4-trichlorobenzene in rat liver

In the present study, we investigated the contribution of methylsulfonyl metabolite derived from 1,2,4-trichlorobenzene (1,2,4-TCB) on the delta-aminolevulinic acid (ALA) synthetase induction by the parent compound in rats. The time courses of increasing of hepatic microsomal total cytochrome P450 content after a single i.p. administration of 1,2,4-TCB (1.36 mmol/kg), and 2,3,5- and 2,4,5-trichlorophenyl methyl sulfones (2,3,5- and 2,4,5-TCPSO2Mes) (50 micromol/kg each) were in parallel with those of increasing of the total heme content in liver microsomes. 1,2,4-TCB significantly increased the heme oxygenase activity, but 2,3,5- and 2,4,5-TCPSO2Mes did not. On the other hand, 1,2,4-TCB and 2,3,5-TCPSO2Me markedly enhanced the ALA synthetase activity. No change was observed in this enzyme activity after the administration of 2,4,5-TCPSO2Me. After the administration of 1,2,4-TCB to the rats treated with DL-buthionine-(S,R)-sulfoximine (BSO) and to the non-BSO-treated rats, the concentrations of both 2,3,5- and 2,4,5-TCPSO2Mes were significantly lower in liver of the BSO-treated rats than in liver of the non-BSO-treated rats. Additionally, the 1,2,4-TCB did not elevate the ALA synthetase activity in the BSO-treated rats. On the other hand, the administration of 2,3,5-TCPSO2Me to BSO-treated rats resulted in induction of ALA synthetase. The results strongly suggest that the methyl sulfone derived from 1,2,4-TCB, i.e., 2,3,5-TCPSO2Me, contributes highly to the induction of the ALA synthetase activity by the parent compound.

The liver, kidney, and thyroid toxicity of chlorinated benzenes

The acute toxicity of a number of chlorinated benzenes, ranging from monosubstituted to pentasubstituted benzenes, was studied in rats. Toxic effects on the liver, the kidneys, and the thyroid were monitored after a single ip administration of 1, 2, or 4 mmol/kg monochlorobenzene (MCB), 1,2-dichlorobenzene (1,2-DICB), 1,4-dichlorobenzene (1,4-DICB), 1,2,4-trichlorobenzene (1,2,4-TRCB), and pentachlorobenzene (PECB). Due to its low solubility, 1,2,4,5-tetrachlorobenzene (1,2,4,5-TECB) was tested at a highest dose of 0.8 mmol/kg. 1,2-DICB and 1,2,4-TRCB produced the most severe hepatotoxic effects when compared with an equimolar dose of the other chlorinated benzenes, as determined by plasma ALT profile and histopathological changes after 72 hr. MCB was considerably less hepatotoxic. Severe degenerative damage to the kidney was only observed in a few rats treated with 1,2,4-TRCB. However, protein droplets in the tubular epithelial cells were observed at 72 hr after administration of 1,4-DICB, 1,2,4-TRCB, 1,2,4,5-TECB, and PECB. In the latter two groups, these protein droplets were still observed 9 days after administration. All chlorinated benzenes tested excluding MCB induced a reduction in plasma thyroxine levels. The extent of decrease in plasma thyroxine was more severe in rats treated with 1,2,4-TRCB or PECB and correlated well with the relative binding affinities of the phenolic metabolites to the plasma transport protein for thyroxine, i.e., transthyretin. The present study indicates that the establishment of a structure-activity relationship with regard to toxicity depends on the sensitivity of the respective target organs. In the series of (poly)chlorinated benzenes studied, ranging from mono- to pentachlorobenzene, the most severe effects on liver, kidney, and thyroid were observed for 1,2,4-substitution.

Metabolic activation of 1,2,4-trichlorobenzene and pentachlorobenzene by rat liver microsomes: a major role for quinone metabolites

Microsomal metabolism of 1,2,4-[14C]trichlorobenzene (1,2,4-TrCB) and [14C]pentachlorobenzene (PeCB) was studied with special emphasis on the conversion-dependent covalent binding to protein and DNA. 1,2,4-TrCB was metabolized to 2,3,6- and 2,4,5-trichlorophenol, and to a lesser extent to 2,4,6- and 2,3,5-trichlorophenol, and trichlorohydroquinone. About 10% of all metabolites became covalently bound to protein in a rather nonselective way. For 1,2,4-TrCB and PeCB a strong correlation between secondary metabolism to hydroquinones and covalent binding was established. Protein binding was completely inhibited by the addition of ascorbic acid, indicating quinone metabolites as the sole reactive species formed. Both 1,2,4-TrCB and PeCB alkylated DNA, although to a much lesser extent than protein (0.5 and 0.3% of all metabolites, respectively). Nonquinone intermediates, presumably epoxides, were responsible for a minor portion of the observed DNA binding, since complete inhibition by ascorbic acid was not reached. The differential role of cytochrome P450 both in primary and in secondary metabolism was demonstrated by the use of microsomes from rats pretreated with different inducers. Dexamethasone (DEX) microsomes (cytochrome P450IIIA1) showed the highest activity toward these chlorinated benzenes (14 nmol/mg/5 min for 1,2,4-TrCB and 36 nmol/mg/10 min for PeCB, both with regard to the formation of phenols and to the formation of protein-bound metabolites. In addition, DEX microsomes preferentially formed 2,3,6-trichlorophenol, whereas other microsomal suspensions formed 2,4,5-trichlorophenol as the major isomer. The present study clearly demonstrates the high alkylating potency of secondary quinone metabolites derived from chlorinated benzenes and poses a need for reevaluation of the role of epoxides in the observed toxicity of these compounds.

The metabolism of pentachlorobenzene by rat liver microsomes: the nature of the reactive intermediates formed

Metabolism of [14C]-pentachlorobenzene by liver microsomes from dexamethasone-induced rats results in the formation of pentachlorophenol and 2,3,4,6-tetrachlorophenol as major primary metabolites in a ratio of 4:1, with 2,3,4,5- and 2,3,5,6-tetrachlorophenols as minor metabolites. The unsubstituted carbon atom is thus the favourite site of oxidative attack, but the chlorine substituted positions still play a sizable role. As secondary metabolites both para- and ortho-tetrachlorohydroquinone are formed (1.4 and 0.9% of total metabolites respectively). During this cytochrome P450-dependent conversion of pentachlorobenzene, 5-15% of the total amount of metabolites becomes covalently bound to microsomal protein. Ascorbic acid inhibits this binding to a considerable extent, indicating that quinone metabolites play an important role in the binding. However, complete inhibition was never reached by ascorbic acid, nor by glutathione, suggesting that other reactive intermediates, presumably epoxides, are also responsible for covalent binding.

Hematologic and hepatic manifestations of the cutaneous porphyrias

This chapter has dealt with five photocutaneous forms of human porphyria. The forms are a diverse group of disorders with many different hematologic, hepatologic, and neurologic manifestations. In essence, most photocutaneous porphyrias occurring in childhood will relate to congenital erythropoietic porphyria or protoporphyria. The nature of the skin lesions and a study of the heme precursor profile in red cells, plasma, urine, and feces should easily distinguish these two conditions. CEP is a disease wherein photomutilation is a dominant concern and aggressive new approaches of therapy also have been discussed. In protoporphyria, the dermatologic problem is less severe and the dermatologist should be aware that a subset of patients could develop active liver disease that may lead to fatal cirrhosis. Novel approaches of therapy have been briefly alluded to. With regard to postpubertal photocutaneous porphyria, the classic porphyria cutanea tarda syndrome is associated with liver disease, usually alcoholic with siderosis, and the treatment by phlebotomy to reduce hepatic iron is highly effective. The potential danger of liver carcinoma has been discussed. In subsets of porphyria cutanea tarda, this can be an endemic disease relating to environmental factors, ie, ingestion of polyhalogenated hydrocarbons. The biochemical diagnosis can be attained by fairly straight-forward solvent extraction analyses of urine and feces, showing the dominance of uroporphyrin excretion in the urine and coproporphyrin in the feces. Chromatographic techniques in plasma, bile, and feces reveal a PCT-specific porphyrin: isocoproporphyrin. Rare subtypes with hematologic manifestations, ie, hepatoerythropoietic porphyria and CEP, indicate the wide spectra of disorders that might be associated with a spontaneous deficiency of uroporphyrinogen decarboxylase activity. These latter syndromes are, however, rare. Two hereditary hepatic porphyrias, ie, autosomal dominantly inherited VP and HCP, have been briefly discussed. The hepatic lesion is metabolic, not morphologic, and its expression by the liver relates to its adaptive response to induction of microsomal hemoproteins by a variety of exogeneous and endogeneous compounds, eg, drugs and hormones. Photocutaneous lesions of HCP and VP are identical to PCT, the latter having no neurologic sequelae. In the former two, however, exposure of persons to drugs, such as the hydantoins and barbiturates, can lead to potentially fatal acute porphyric attacks.(ABSTRACT TRUNCATED AT 400 WORDS)

Exposure to toxic agents: the heme biosynthetic pathway and hemoproteins as indicator

The heme biosynthetic pathway is closely controlled by levels of the end product of the pathway, namely, heme, and porphyrins are normally formed in only trace amounts. When control mechanisms are disturbed by xenobiotics, porphyrins accumulate and serve as a signal of the interaction between a xenobiotic and the heme biosynthetic pathway. For example, an increase in erythrocyte protoporphyrin is a useful measurement for early detection of exposure to lead and porphyrinuria was an early manifestation of a hexachlorobenzene-induced porphyria in Turkey. In recent years a variety of additional xenobiotics has been shown to interact with the heme biosynthetic pathway, namely, halogenated aromatic hydrocarbons, pesticides, sulfides, and a variety of metals. Moreover, different xenobiotics (e.g., dihydropyridines and compounds containing unsaturated carbon-carbon bonds) interact with the prosthetic heme of cytochrome P-450 forming novel N-alkylporphyrins.

Toxic responses to acute, subchronic, and chronic oral administrations of monochlorobenzene to rodents

Acute (single exposure), 14-d repeated exposure, 91-d subchronic, and 103-wk chronic toxicity studies of orally administered (gavage, in corn oil) monochlorobenzene were conducted in male and female Fischer-344 rats and B6C3F1 hybrid mice. A single exposure to 4000 mg/kg was lethal to male and female rats, while a single exposure to a dose as low as 1000 mg/kg was lethal to mice. Fourteen daily exposures to 1000 mg/kg caused death in rats of both sexes, but neither survival nor clinical health were compromised at 500 mg/kg in rats or mice. In the 91-d studies, wherein monochlorobenzene was administered once daily, 5 d/wk, survival was reduced by doses of 500 mg/kg and higher in rats, and by doses of 250 mg/kg and higher in mice. Dose-dependent necrosis of the liver (hepatocytes), degeneration or focal necrosis of the renal proximal tubules, and lymphoid or myeloid depletion of the spleen, bone marrow, and thymus (mild to severe) were produced by doses of 250 mg/kg or greater of monochlorobenzene in both sexes of rats and mice, although the incidences of these lesions varied considerably by sex and species. Consistent changes in the circulating blood components were not observed, but a mild porphyrinuria was detected at the higher doses. No toxic effects were observed at doses of 125 mg/kg or less. In the 2-yr studies, wherein monochlorobenzene was administered once daily, 5 d/wk, doses of 30 or 60 mg/kg in male mice and 60 or 120 mg/kg in female mice and male and female rats did not produce any evidence of toxicity. Doses of 60 or 120 mg/kg caused slight (statistically significant at 120 mg/kg; p less than 0.05) increases in the frequencies of male rats with neoplastic nodules of the liver. Increased tumor frequencies were not observed in female rats or in male or female mice receiving monochlorobenzene.

Mechanism of hexachlorobenzene-induced porphyria in rats. Effect of phenobarbitone pretreatment

The effect of a pretreatment with phenobarbitone (PB) on the porphyrinogenic action exerted by hexachlorobenzene (HCB) was examined in female rats. Kinetic studies of enzyme function after HCB poisoning showed that porphyrinogen carboxy-lyase was the only enzyme of haem biosynthesis that markedly lowered its activity. Both stages of uroporphyrinogen (UPG) III decarboxylation were decreased. This enzyme, together with UPG I synthase (increased levels) were the first enzymes altered. Subsequently, an increase in delta-aminolaevulinate (AmLev) synthase and ferrochelatase was detected; AmLev dehydratase was the last to increase. On long-term exposure, PB alone did not modify the basal values of haem intermediates; only the content of cytochrome P-450 increased. All the enzyme activities studied showed no significant changes, except ferrochelatase, which increased. With both drugs the metabolic impairment promoted by HCB was accelerated and enhanced by prior PB treatment leading to the onset of an earlier and stronger porphyria. A more noticeable accumulation and excretion of higher carboxylated porphyrins and precursors was more promptly observed as a consequence of the early porphyrinogen carboxy-lyase blockade and the concomitant induction of AmLev synthase. Although the enzymic activities of both AmLev dehydratase and ferrochelatase were enhanced, this response differed in time. For UPG I synthase this pretreatment elicited lower values than those found in the HCB group. Cytochrome P-450 contents were immediately and slightly enhanced by all the drugs, but the values for the combined treatment were the lowest. Of the several hypotheses that could explain the action of HCB on the haem pathway, our results would suggest that the porphyrinogenic action of HCB is mediated by some of its metabolic products.

Studies on the porphyrinogenic action of 1,2,4-trichlorobenzene in birds

The porphyrinogenic action of 1,2,4-trichlorobenzene (TCB) was examined in 17-day-old embryos, day-old chicks, 18-day-old chickens and adult Japanese quail. The quail was found to be the most sensitive species towards TCB induced porphyria whereas the chick embryo was totally non-responsive. The liver porphyrins of Japanese quail were increased in a dose-dependent manner 1 day after TCB. Elevation in porphyrin levels in quail was associated with comparable increases in delta-aminolevulinic acid synthetase (ALA-S) activity 1 day after TCB treatment. In contrast, ferrochelatase activity was found to be unchanged 1 day after TCB. Multiple administration of TCB produced only a slight increase in liver porphyrin levels and ALA-S activity in quail. However, there was a marked induction in ferrochelatase activity suggesting increased porphyrin turnover. Liver glutathione and glutathione S-transferase activity were also significantly increased following repeated administration of TCB in quail, which could indicate an enhancement of detoxication of reactive metabolites of TCB. Thus, it is suggested that the inability of low multiple doses of TCB to cause porphyria in Japanese quail may be related to the low responsiveness of ALA-S but high inducibility of ferrochelatase liver GSH and glutathione S-transferase.

Subchronic inhalation toxicity of 1,3,5-trichlorobenzene

Male and female rats were exposed to 0, 10, 100 or 1000 mg/m3 of 1,3,5-trichlorobenzene vapors for 6 hours daily, 5 days a week, for up to 13 weeks. After 4 and 13 weeks of exposure, animals were sacrificed and examined for changes in blood, clinical chemistry, internal organs, and tissues resulting from the 1,3,5-trichlorobenzene treatment. No treatment-related effects on the blood and clinical chemistry were evident. The only effects that were considered treatment-related were a squamous metaplasia and hyperplasia in the respiratory epithelium in the nasal passages of high-dose rats and the increased incidence of dried red material on the faces of these rats during exposures compared with other groups.

Subchronic dermal toxicity study of trichlorobenzene in the rabbit

Groups of five male and five female New Zealand albino rabbits were treated by skin application with either 0 (distilled water control), 30, 150 or 450 mg/kg undiluted trichlorobenzene (ICB) for 5 days/week for four weeks. No treatment related systemic effects were observed at any of the treatment levels when body weight, clinical chemistry and organ weight parameters were measured. Systemic effects due to dermal application of TCB were present only in rabbits given 450 mg/kg/day. These effects included a slight but statistically significant increase in the urinary coproporphyrin excretion in males and slight pallor of the liver at gross necropsy in both sexes. Localized effects at the site of application were present in all treated rabbits. These effects were characteristic of the response to dermal irritation. Grossly, the fur was matted by a fine white bran-like scales with variable degrees of erythema, fissures, erosions and ulcers. Histopathologically, there was inflammation and thickening of the epidermis. The size of the affected area varied directly with the dose level. Based on the results of the study, it was concluded that a dose level of 450 mg/kg/day of TCB applied dermally to rabbits induced slight systemic toxicity. The no-observable effect level for systemic toxicity was 150 mg/kg/day when TCB was applied to the skin of male and female rabbits over the course of 30 days.

Effects of iron-loading and ethanol treatment on rat porphyrin metabolism

Rats were fed for 50 weeks with a standard diet containing 5.9% ferric ammonium sulphate. Half of these animals drank normal water, and the other half water containing 5% ethanol (groups 1 and 2). Two other groups received normal food, but drank water containing 5 or 10% ethanol (groups 3 and 4) for 40 weeks. Histologic examinations revealed that the iron-loading resulted in only mild hepatic siderosis in groups 1 and 2, the degree of siderosis not differing appreciably in the two groups. The ethanol led to fatty degeneration too in the liver of animals in group 2. Both iron-loading and ethanol treatment, either separately or in combination, increased the porphyrin excretion, but the distribution of the various porphyrins in the urine and faeces showed merely the symptoms of an aspecific poisoning. A significantly elevated uroporphyrin excretion was not observed in any of the groups, and thus the results support the view that dietary iron-loading and ethanol consumption can not be regarded as direct aetiologic factors in the pathomechanism of porphyria cutanea tarda. At the same time, the results suggest that vitamin E therapy, frequently employed effectively in porphyria cutanea tarda, can not be considered a causal intervention as regards the mechanism of action.

Non-mutagenicity for Salmonella of the chlorinated hydrocarbons aroclor 1254, 1,2,4-trichlorobenzene, mirex and kepone

A polychlorinated biphenyl mixture, Aroclor 1254, two commercial grade insecticides, mirex and kepone, and a pesticide breakdown product, 1,2,4-trichlorobenzene were evaluated for mutagenicity and hepatic enzyme induction potential in the Salmonella/microsomal assay. None was found to revert strains TA1535, TA1537, TA98 or TA100 when tested with or without metabolic activation. Liver microsomal extracts (S9) from rats induced with 1,2,4-trichlorobenzene were shown to differ from S9 of either control or Aroclor 1254-induced rats in the capacity to activate 2-aminoanthracene mutagenesis.

Brominated benzene induction of hepatic porphyria

Unlike the highly porphyrinogenic fungicide hexachlorobenzene, hexabromobenzene was a poor inducer of porphyria. Similarly, 1,2-dibromobenzene and 1,2,4-tribromobenzene, while causing small increases in hepatic porphyrins, did not increase ALA synthetase or the urinary excretion of porphobilinogen (PBG), aminolevulinic acid (ALA) or porphyrins.

[Hexachlorbenzene (HCB) induced porphyria in rats. Influence of HCB-metabolites on the biosynthesis of heme (author's transl)]

Female adult Wistar rats were fed with a diet containing 0.05% hexachlorobenzene (HCB) or its metabolites, pentachlorobenzene (PCB) and pentachlorphenole (PCP). These chlorinated aromatic hydrocarbons produced an increase in the liver cytochrome P-450 content in about the same degree, however, only the application of HCB showed an extremely high rise in the P-450 enzymatic activity expressed in terms of the O-dealkylation of 7-Ethoxycoumarine. No alteration was observed in the urinary porphyrin excretion in the PCB and PCP treated animals, whereas 60 days after the beginning of the HCB application a high level of porphyrins could be detected in the urine of the animals. It seems unlikely therefore that the HCB metabolites (PCB and PCP) are porphyrogenic agents. In addition, although induction of the liver cytochrome P-450 system was observed after PCP pretreatment of the rats over a period of 40 days, the consequent application of HCB did not influence the establishment of the experimental porphyria.

Chlorinated benzene induction of hepatic porphyria

1,4-Dichlorobenzene and 1,2,4-trichlorobenzene were compared with hexachlorobenzene which is known to cause porphyria. Although hexachlorobenzene administration resulted in a manyfold increase in liver porphyrin levels and urinary excretion of porphyrins, the lesser chlorinated compounds did not do so.

[Long time HCB exposition of rats: influence on the porphyrin excretion in the urine and on the cytochrome P-450 in the liver (author's transl)]

Adult female Wistar rats were fed with a diet containing 0.05% HCB. About the 55th day of the experiments an increase of porphyrin and its precursors (ALA and PBG) in the urine of the rats can be measured. In contrast the induction of the O-dealcylation reaction of 7-ethoxycoumarin could be already measured 2 days after starting with the HCB feeding. The inhibition of the enzymatic reaction by metyrapone, naphthoflavone, tetrahydrofurane or CO/O2 presents neither the typical pattern of the phenobarbital type nor the typical pattern of the benzpyrene type of cytochrome P-450. When the animals became porphyric (about the 55th day of the HCB exposure) no qualitative changes in the cytochrome P-450 pattern could be demonstrated. The relationship between the enzyme induction in the liver and the porphyria following the HCB application is still remaining unclear.