Effects of dietary antioxidants on the biotransformation and porphyrinogenic action of hexachlorobenzene in two strains of rats

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.

Section: Contentssupporting

confidence: 76%

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

Calmanovici¹,

Molina²,

Yamasato³

et al. 1984

“…(1981) because of the protection afforded by iron depletion and by dietary butylated hydroxyanisole (Sweeney, 1982). We have been unable to find any protective effect of this antioxidant (similar results have been reported for rats ;Debets et al, 1981), but the great enhancement by iron overload of both HCB and TCDD porphyrogenicity in mice (Smith & Francis, 1983;Greig et al, 1984) would be consistent with a related mechanism perhaps involving interaction between microsomal electron transport and ferritin (De Matteis & Stonard, 1977;Rowley & Sweeney, 1984). Siderotic mice fed HCB produce more lipid peroxidation products than do controls (Smith et al, 1986b) but the finding of a marked difference between hepatic microsomes from C57BL/10 and DBA/2 mice in the production of malondialdehyde, the oxidation of NADPH and 02 consumption, may be of greater relevance despite the lack of induction by HCB.…”

Section: Mechanism Of the Inhibition Of Uroporphyrinogen Decarboxylasesupporting

confidence: 61%

Mechanistic studies of the inhibition of hepatic uroporphyrinogen decarboxylase in C57BL/10 mice by iron-hexachlorobenzene synergism

Smith¹,

Francis²,

Kay³

et al. 1986

Porphyria was induced in C57BL/10 mice with iron overload by a single oral dose (100 mg/kg) of hexachlorobenzene (HCB). Within 2 weeks hepatic uroporphyrinogen decarboxylase (EC 4.1.1.37) was inhibited, reaching a maximum (greater than 95%) at 6-8 weeks. There was no recovery by 14 weeks, despite a fall in liver HCB concentrations to only 6% of the day-3 value. The major rise in hepatic porphyrin levels occurred after 4 weeks and secondary inhibition of uroporphyrinogen synthase (EC 4.2.1.75) was inferred from the progressively greater proportion of uroporphyrin I present relative to the III isomer. Plasma alanine aminotransferase (EC 2.6.1.2) activity was also elevated. Although, in further studies, total microsomal cytochrome P-450 content and ethoxyphenoxazone de-ethylase activity reached a peak a few days after dosing and had declined significantly at the time of maximum inhibition of the decarboxylase, additional treatment of HCB-dosed mice with a cytochrome P1-450 inducer, beta-naphthoflavone, enhanced the inhibition, whereas piperonyl butoxide, an inhibitor of cytochrome P-450, partially protected. Uroporphyrinogen decarboxylase was not radiolabelled in vivo by [14C]HCB. There was no major difference in the ability to hydroxylate HCB between hepatic microsomes from induced C57BL/10 mice and those from the insensitive DBA/2 strain. By contrast, lipid peroxidation, in the presence of NADPH, was 8-fold greater in control C57BL/10 microsomes than in DBA/2 microsomes and was stimulated by iron treatment (although not by HCB). The results suggest that the inhibition of hepatic uroporphyrinogen decarboxylase is unlikely to be due to a direct effect of a metabolite of HCB but to another process requiring a specific cytochrome P-450 isoenzyme and an unknown iron species.

“…Differences between the groups were not statistically significant. (Debets et al, 1981;de Verneuil et al, 1983;Wainstok de Calmanovici et al, 1984). The structure and metabolism of MC and polyhalogenated porphyrogens are different and MC did not cause porphyria in the absence of ALA ( Fig.…”

Section: Discussionmentioning

confidence: 97%

“…Two different explanations for the relationship between cytochrome P-450 induction and the development of uroporphyria have been proposed that are compatible with this type of enzyme defect. First, polyhalogenated hydrocarbons may be converted by a cytochrome P-450-dependent reaction to metabolites which bind to the active site of the enzyme (Debets et al, 1981 ;de Verneuil et al, 1983;Wainstock de Calmanovici et al, 1984). Secondly, polyhalogenated hydrocarbons may interact with cytochrome P-450 to stimulate the production of reactive oxygen species which inhibit uroporphyrinogen metabolism by substrate oxidation, inactivation of the enzyme or some other oxidative process that is iron dependent (Ferioli et al, 1984;Elder et al, 1986;Sinclair et al, 1986Sinclair et al, , 1987.…”

Section: Introductionmentioning

confidence: 99%

Uroporphyria produced in mice by 20-methylcholanthrene and 5-aminolaevulinic acid

Urquhart

Elder

Roberts

et al. 1988

Iron-loaded male C57BL/6 mice allowed free access to an aqueous solution of 5-aminolaevulinic acid (ALA) (2 mg/ml) as their only drink, develop severe uroporphyria within 9 days of a single intraperitoneal dose of 20-methylcholanthrene (MC) (125 mg/kg). At 21 days, uroporphyrinogen decarboxylase (EC 4.1.1.37) activities are less than 10% of control activities. The porphyria is not dependent on pretreatment with iron and persists for at least 21 days after withdrawal of ALA. The same intraperitoneal dose of MC does not produce porphyria within 21 days when given without ALA. Continuous administration of ALA markedly accelerates the onset of porphyria in iron-loaded male C57BL/6 mice after a single intraperitoneal dose of hexachlorobenzene (200 mg/kg); mice given phenobarbitone and ALA do not become porphyric. MC with ALA does not produce porphyria in iron-loaded male DBA/2 mice. At least two separate events are needed to produce uroporphyria in mammals: induction of a specific form of cytochrome P-450 and stimulation of the formation of intermediates of haem biosynthesis in the liver. These results show that severe, persistent porphyria can be produced in mammals by compounds other than polyhalogenated aromatic hydrocarbons and suggest that a similar mechanism underlies the porphyrogenic action of halogenated and non-halogenated compounds.