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

Melatonin formation in pineal gland from rats with hexachlorobenzene experimental porphyria

Hexachlorobenzene produces an experimental hepatic porphyria in rats, which is similar to human porphyria cutanea tarda, with hyperpigmentation as one of its characteristic features. Alterations in tryptophan metabolism have been previously observed in this chronic porphyria. Melatonin formation from tryptophan via serotonin shows diurnal rhythmicity in the pineal gland, and higher values are observed during the dark phase of an imposed light-dark cycle. The purpose of this study was to determine the contents of tryptophan and its metabolites in pineal gland of normal and hexachlorobenzene-treated rats in order to find alterations potentially related to porphyria cutanea tarda. Results show that in animals with this experimental porphyria some tryptophan metabolite levels (serotonin and 5-hydroxyindoleacetic acid) increase only during the light period, whereas tryptophan content remained equal to the controls. Hydroxyindole-O-methyltransferase activity also increases by light in pineal gland from hexachlorobenzene-treated rats. On the other hand, tryptophan is converted to melatonin in the dark period, but this route is not exacerbated in hexachlorobenzene porphyria. The relevance of these alterations is discussed in relation to hyperpigmentation, neoplastic and oxidative stress processes associated with this porphyria.

Hexachlorobenzene as hormonal disruptor--studies about glucocorticoids: their hepatic receptors, adrenal synthesis and plasma levels in relation to impaired gluconeogenesis

In Wistar rats, hexachlorobenzene (HCB) depresses the gluconeogenic enzyme phosphoenolpyruvate-carboxykinase (PEPCK). In the liver, glucocorticoids (GC) normally regulate the glucose synthesis by acting on PEPCK. Thus, the aim of this work was to investigate, in a time-course study, the effects of HCB on plasma GC, its adrenal synthesis and stimulation, and the kinetic parameters of its hepatic receptors (GR) in relation to the gluconeogenic blockage produced by HCB. Plasma corticosterone (CORT) concentration, urinary porphyrins and hepatic PEPCK were determined after 2, 4, 6 and 8 weeks of HCB-treatment. The effect of HCB on kinetic parameters of GR was studied in adrenalectomized porphyric rats after 2, 4 and 8 weeks of treatment. Additionally, adrenal CORT synthesis in the same weeks was measured with or without ACTH. Results show that plasma CORT in intoxicated animals dropped significantly after 2 and 4 weeks of treatment (23% and 58%, respectively), and then remained constant until the 8th week. HCB also promoted a reduction in the number of hepatic GR (50-55%) without modifying affinity. After 8 weeks, when porphyria was well established (40-50-fold increase in urinary porphyrins), a reduction (52%) in hepatic GR number, as well as a decrease in PEPCK activity (56%) were observed. Moreover, CORT biosynthesis in adrenals from intoxicated animals significantly decreased (60%) without changes in ACTH effect. Briefly, this paper shows that HCB causes a disruption in GC and GR. This disturbance could contribute to the negative effect on glucose synthesis through PEPCK regulation, thus modulating porphyria. These results enhance the knowledge about the hormonal disruption produced by chlorinated xenobiotics.

Glycogen metabolism and glucose transport in experimental porphyria

Hexachlorobenzene (HCB) is a fungicide of well-known porphyrinogenic ability, which induces an experimental porphyria that resembles human porphyria cutanea tarda (PCT) in several animal species. It has been demonstrated that high glucose ingestion prevents porphyria development, and high-fat/high-protein diets enhance HCB porphyrinogenic ability. On the contrary, a diet rich in carbohydrates reduces HCB effects. The aim of this work was to study HCB effects on glycogen synthesis and degradation, as well as on glucose synthesis and transport, in order to elucidate whether would justify the beneficial use of carbohydrates in this porphyria. Rats were treated with HCB dissolved in corn oil (five daily doses 100mg/kg body weight). Results showed that: (1) HCB caused an increase in glycogen content; (2) glycogen synthase activity increased three times, and phosphorylase activity decreased about 40% due to fungicide intoxication. The effect of HCB on these two activities accounted for the higher glycogen content observed in treated animals; (3) three gluconeogenic enzymes were reduced 30-50%; (4) glucose uptake in the liver decreased in all weeks studied. The alterations found in glucose synthesis, its uptake in liver and other tissues, and its release from glycogen might contribute to the biochemical porphyria picture and would account for the effect of glucose above mentioned.

Tryptophan metabolism via serotonin in rats with hexachlorobenzene experimental porphyria

One of the three pathways for the metabolisation of dietary tryptophan is the formation of serotonin. Tryptophan hydroxylase catalyses the formation of 5-hydroxytryptophan, the first and regulatory step of this biosynthesis. The aim of the present work is to study alterations in this tryptophan metabolism in rats with experimental Porphyria Cutanea Tarda induced by hexachlorobenzene. With this purpose, the content of tryptophan and its metabolites related to the serotonin pathway are determined by HPLC techniques, in tissues (brain, liver and gut) and in fluids (blood, plasma and urine) of controls and hexachlorobenzene-porphyric rats. In these experimental-porphyric animals, we determine a significant increase in the excretion of 5-hydroxyindole acetic acid in urine and a decrease in the content of serotonin in small gut, respect to controls. Significant increases in contents of serotonin in 24-hr urine and tryptophan in liver are also found. No other significant variations for the different metabolites are detected in any of the tissues and fluids studied. Brain and liver activities of the rate-limiting enzyme tryptophan hydroxylase can only be measured in porphyric rats. Our results agree with an increased turnover of gastrointestinal serotonin derived from dietary tryptophan and its excretion as urinary 5-hydroxyindole acetic acid, which is formed in liver. An increased serotonin pathway in porphyric livers is confirmed by the measured increase in the activity of hepatic tryptophan hydroxylase. The absence of neurological symptoms in patients with Porphyria Cutanea Tarda could be related to the absence of a statistically significant variation in serotonin content shown in brain.

The decrease in uroporphyrinogen decarboxylase activity induced by ethanol predisposes rats to the development of porphyria and accelerates xenobiotic-triggered porphyria, regardless of hepatic damage

We evaluated the porphyrinogenic ability of ethanol (20% in drinking water) per se, its effect on the development of sporadic porphyria cutanea tarda induced by hexachlorobenzene in female Wistar rats (170-190 g, N = 8/group), and the relationship with hepatic damage. Twenty-five percent of the animals receiving ethanol increased up to 14-, 25-, and 4.5-fold the urinary excretion of delta-aminolevulinate, porphobilinogen, and porphyrins, respectively. Ethanol exacerbated the precursor excretions elicited by hexachlorobenzene. Hepatic porphyrin levels increased by hexachlorobenzene treatment, while this parameter only increased (up to 90-fold) in some of the animals that received ethanol alone. Ethanol reduced the activities of uroporphyrinogen decarboxylase, delta-aminolevulinate dehydrase and ferrochelatase. In the ethanol group, many of the animals showed a 30% decrease in uroporphyrinogen activity; in the ethanol + hexachlorobenzene group, this decrease occurred before the one caused by hexachlorobenzene alone. Ethanol exacerbated the effects of hexachlorobenzene, among others, on the rate-limiting enzyme delta-aminolevulinate synthetase. The plasma activities of enzymes that are markers of hepatic damage were similar in all drug-treated groups. These results indicate that 1) ethanol exacerbates the biochemical manifestation of sporadic hexachlorobenzene-induced porphyria cutanea tarda; 2) ethanol per se affects several enzymatic and excretion parameters of the heme metabolic pathway; 3) since not all the animals were affected to the same extent, ethanol seems to be a porphyrinogenic agent only when there is a predisposition, and 4) hepatic damage showed no correlation with the development of porphyria cutanea tarda.

Effects of the porphyrinogenic compounds hexachlorobenzene and 3,5-diethoxycarbonyl-1,4-dihydrocollidine on polyamine metabolism

The naturally occurring polyamines--putrescine, spermidine and spermine--are organic cations present in all living cells and essential for cell growth and differentiation. The aim of the present study was to extend the investigations on the effects of porphyrinogenic compounds on polyamine metabolism. This was achieved by studying putrescine, spermidine and spermine levels in a model of acute porphyria, i.e. 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-induced porphyria, and in a model of non-acute porphyria, i.e. hexachlorobenzene (HCB)-induced porphyria. HCB administration to female Wistar rats for 7, 14, 21, 28 and 56 days did not alter polyamine levels in liver, even though rats presented clear signs of HCB-induced porphyria. In contrast to HCB, DDC treatment resulted in a remarkable increase in putrescine levels in the liver of female and male Sprague-Dawley rats. This increase was due, at least in part, to ornithine decarboxylase (ODC) activation. DDC induction of putrescine levels did not show organ specificity, since it could also be seen in adrenal gland. Interestingly, the deregulation of polyamine biosynthesis occurred concomitantly with the deregulation of the heme biosynthetic pathway. In addition to porphyria, it is known that DDC intoxication affects several proteins of the hepatocyte cytoskeleton. It is suggested that DDC-induced increase in ODC activity and putrescine levels may be an early event contributing to alter the cytoskeleton.

Heme metabolism after discontinued hexachlorobenzene administration in rats: possible irreversible changes and biomarker for hexachlorobenzene persistence

The aim of the present study was to determine whether short-term administration of hexachlorobenzene (HCB) (1 g/kg body wt., suspended in water, 5 days/week), could cause and maintain marked porphyria in the absence of the exogenous drug, and whether porphyria parameters can be useful as biomarkers of HCB persistence in rats. Hepatic uroporphyrinogen decarboxylase activity, its inhibitor formation, porphyrin content and composition were studied in Wistar rats treated with the fungicide for 1, 2, 3, or 4 weeks and then withdrawn for a 20-week period. The time course of urinary porphyrin excretion was studied for 7 weeks either by continuous treatment for the entire period, or a 1-week HCB administration. The degree of porphyria achieved by rats after 20 weeks of suspended HCB administration was severe, independent of the length of the treatment, and even higher than that observed in animals analysed immediately at the end of each treatment. Rats treated with HCB for 1 week showed a modest decrease in uroporphyrinogen decarboxylase and low inhibitor formation, and exhibited a greater enzyme inhibition, inhibitor formation, hepatic porphyrin accumulation, and an altered pattern of porphyrin composition in the absence of the exogenous drug. Independent of the treatment, urinary porphyrins rose after a delay of 5 weeks. Substantial amounts of HCB were still found in fat of rats treated with HCB for 1 week, after a withdrawal period of 20 weeks. These results suggest that the high persistence of HCB in tissues acts as a continuous source of the xenobiotic, and stimulus for heme biosynthesis derangement. The alterations induced by HCB within 1 week of treatment could be regarded as an initial trigger for irreversible damage on heme metabolism. Thus, abnormalities in heme biosynthesis can be considered effective markers of HCB persistence in rats or of irreversible HCB-induced damage. Taking into account the delayed and enhanced metabolic effects of HCB, it is advisable that porphyria parameters should be evaluated not only immediately after exposure, but also some time afterwards, especially in susceptible and occupationally-exposed populations.

Hexachlorobenzene-induced alterations on neutral and acidic sphingomyelinases and serine palmitoyltransferase activities. A time course study in two strains of rats

Hexachlorobenzene (HCB) induces porphyria both in humans and rodents, and hepatocarcinoma in rodents. In a previous work we observed that HCB produces a continuous decrease in hepatic sphingomyelin (SM) content in Wistar rats. A distinguishing characteristic of sphingolipids breakdown products is their participation in anti-proliferative and apoptotic processes and in the suppression of oncogenesis. As a first step to elucidate the role of SM decrease in the hepatotoxicity induced by HCB, the present study evaluates the metabolic causes of the continuous decrease in hepatic SM content observed in Wistar rats with HCB intoxication, and its relation with porphyria development. For this purpose, the time-course (3, 7, 15, 21 and 28 days) of the effects of HCB on hepatic SM levels and on some of the enzymes of SM synthesis (serine palmitoyltransferase, SPT) and catabolism (sphingomyelinases, SMases) was followed, using two strains of rats differing in their susceptibility to acquire porphyria: Chbb THOM (low) and Wistar (high). HCB (1 g kg(-1) b.w. per day) was administered by gastric intubation as an aqueous suspension. After 5 days of HCB treatment, animals were allowed a 2-day recovery period without HCB administration. Two phases in the HCB-induced damages to sphingolipid metabolism were observed. The first stage (7 days of treatment), common to both strains of rats, was characterized by a decrease in hepatic SM levels (17-25%) and in SPT activity (50-43%), while strain differences were found for the later stage. In Chbb THOM rats, hepatic SM content was restored to normal values concomitantly with an increase in SPT activity (44%, at day 28), and without any increase in SM catabolism. In addition, the level of the other phospholipids was not altered. In Wistar rats, hepatic SM levels decreased continuously throughout the experiment, accompanied by increases in SPT, acidic sphingomyelinase (A-SMase) and neutral sphingomyelinase (N-SMase) activities (86, 28.5 and 78% increase, respectively). A role for glutathione (GSH) in the interstrain differences or a direct effect of HCB on SM metabolism was not found. The present study: (a) demonstrates that N-SMase, A-SMase, and SPT are some of the enzymes that play a role in the HCB-induced decrease of hepatic SM content; (b) finds that HCB-induced alterations of SM metabolism do not correlate with HCB-induced accumulation of hepatic porphyrins; and (c) proposes a link between HCB-induced alterations in phospholipid pattern and in SM metabolism. The increased SM hydrolysis produced as a consequence of SMases induction could be regarded as a cellular response to liver injury elicited by HCB, perhaps acting through the activation of SM signal transduction pathway delaying the proliferative processes observed after long-term treatment with HCB in some rodent species. However, such protective mechanism appears to be strain-dependent.

Reproductive effects of hexachlorobenzene in female rats

Hexachlorobenzene (HCB) is a polyhalogenated aromatic hydrocarbon widely distributed in the environment. In animal testing, HCB has been shown to be a reproductive toxin. Previous investigations of the effects of HCB on ovarian function have yielded equivocal results. Thus, the effects of chronic administration of HCB (1 g kg(-1) body wt.) on the ovary and pituitary hormone levels, hepatic and uterine oestradiol receptors, ovarian histopathological changes and oestrus cycle characteristics were investigated in spontaneously cycling rats. Our data demonstrate that HCB treatment, under the conditions of the present study, reduced circulating levels of oestradiol and prolactin without differences in serum concentrations of progesterone. Follicle-stimulating hormone serum levels were elevated. Hexachlorobenzene treatment resulted in irregularity of cycles, characterized mainly as prolonged periods of oestrus with a reduced number of ova recovered. In addition, HCB administration resulted in significantly decreased uterine nuclear oestrogen receptor levels. Histopathological examination revealed degenerative changes of the ovarian follicles and germinal epithelium and increased numbers of atresic follicles.

Studies on the mechanism of uroporphyrinogen decarboxylase inhibition in hexachlorobenzene-induced porphyria in the female rat

Hexachlorobenzene (HCB)-induced porphyria occurs in female, but not male, rats after a delay of 35 days following HCB treatment. Uroporphyrinogen decarboxylase (UROD) inhibition has been proposed as a primary causative event. To determine whether there also exists a delay phase and a sexual dimorphism for UROD inhibition, groups of male and female rats were given HCB (100 mg/kg/day) from Days 1 to 5. Hepatic uroporphyrin III was markedly increased only after Day 33. Liver cytosol UROD activity in HCB-treated female rats with porphyria at Days 33, 40, 47, 54, and 100 was decreased by over 70% compared to concurrent control, whereas treated male rats as well as nonporphyric female rats had UROD activity comparable to control levels at Days 6, 12, 19, 26, 33, 40, 47, and 54. Level of immunoreactive UROD in cytosol of porphyric rats was not modified by HCB. No gender-related differences in liver cytosol radiolabel level ([14C]HCB given as the fifth dose) were found at Days 6 and 30. Chromatography of liver cytosol showed nonspecific binding of radiolabel to proteins for males, porphyric and nonporphyric females, and loss of UROD activity did not correlate with the amount of radiolabel in the UROD-containing fractions. Thus, the gender-specific decrease in UROD activity observed when porphyria develops in female rats (delay of about 4 weeks), as well as the persistence of low activity and porphyria for months, suggests that UROD inhibition was causally related to porphyria.

Hepatic indices of thyroid status in rats treated with hexachlorobenzene

The functional thyroid status of hexachlorobenzene (HCB)-treated rats was studied. HCB caused a depletion of serum thyroxine (T4), but did not change L-3,5,3'-triiodothyronine (T3) levels in serum of rats. The activities of the thyroid regulated mitochondrial enzyme L-glycerolphosphate dehydrogenase (LGPD) and cytosolic enzymes, malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PD) and 6-phosphogluconate dehydrogenase (6PGD) were assayed in livers of normal and HCB (100 mg/100 g bw) treated Wistar rats. Mitochondrial LGPD activity did not change significantly, however ME, 6GPD and G6PD were induced by HCB only in non-thyroidectomized animals. The absence of cytosolic enzymes induction in thyroidectomized rats treated with HCB indicates that HCB is not intrinsically thyromimetic. The induction of hepatic ME, G6PD and 6PGD activities in HCB thyroidectomized rats was dependent on the presence of thyroid hormone. The unchanged activity of mitochondrial LGPD in contrast to the increased activities of the cytosolic enzymes ME, G6PD and 6PGD is not consistent with a shift in functional thyroid status following HCB treatment.

Studies on the active centre(s) of rat liver porphyrinogen carboxy-lyase. In vivo effect of hexachlorobenzene on decarboxylation site(s) of porphyrinogens

1. The role of histidine on the decarboxylation of porphyrinogens of 7-, 6-, and 5-COOH III brought about by porphyrinogen carboxy-lyase (PCL) was studied. 2. For this purpose hepatic PCL from normal and hexachlorobenzene (HCB) treated rats were modified with diethylpyrocarbonate. 3. The results indicated that the enzyme from both normal and porphytic animals had histidine at the binding sites of all the porphyrinogens assayed. 4. Comparative studies between the enzyme from normal and porphyric rats suggested that in vivo HCB treatment affected the active site for the decarboxylation of 7-, 6- and 5-COOH porphyrinogens III at histidine residues. 5. On the other hand arginine modification by 2,3-butanedione treatment altered 5-COOH porphyrinogen III decarboxylation for both enzymes. However this amino acid was not involved at the binding site of this substrate.

Modulation of hexachlorobenzene-induced hepatic porphyria by methyl isobutyl ketone in the rat

Potential toxic interaction between hexachlorobenzene (HCB) and methyl isobutyl ketone (MiBK) was investigated using two different schedules of toxicant administration. The first schedule involved simultaneous administration of HCB (50 mg/kg/d, p.o. in 10 ml/kg corn oil at 10.00 a.m. for 5 d/wk) and MiBK (7.5 mmol/kg/d, p.o. in 10 ml/kg corn oil at 4.00 p.m. for 3 d/wk) for 6 weeks. The second schedule involved an initial dosing of 25 or 50 mg HCB/kg/d for 12 consecutive days, followed by the administration of 7.5 mmol MiBK/kg every other day for 27 days. When administered simultaneously, MiBK reduced the severity of HCB-induced porphyria, but when given sequentially after HCB accumulation, it enhanced the porphyrinogenic response. These results suggest that the effect of combined exposure to HCB and MiBK on hepatic porphyria depends on the sequence of the administration of both chemicals, and that the mechanism involved in this interaction may invoke both the induction and inhibition of specific hepatic isoenzymes by MiBK.

Influence of hepatic tumors caused by diethylnitrosamine on hexachlorobenzene-induced porphyria in rats

The response of female BDVI rats bearing diethylnitrosamine(DENA)-induced hepatic tumors to the porphyrinogenic action of hexachlorobenzene (HCB) was studied. (1) The heme pathway operates in these tumors but they were less affected by HCB than the liver. (2) Tumors did not accumulate porphyrins although the surrounding liver accumulated more porphyrins than livers treated with HCB. (3) DENA/HCB livers which developed a well defined tumor showed slightly less porphyrinogen carboxylyase inhibition and delta-aminolaevulinate synthase induction than HCB rats. (4) The results of the present work suggest that endogenously formed porphyrins would be unable to be accumulated by DENA-induced tumors when the tumoral development precedes the onset of the porphyria.

Effects of hexachlorobenzene and its metabolites pentachlorophenol and tetrachlorohydroquinone on serum thyroid hormone levels in rats

Effects of administration of equimolar doses of hexachlorobenzene (HCB) and its metabolites pentachlorophenol (PCP) and tetrachlorohydroquinone (TCHQ) on serum thyroxine (TT4) and triiodothyronine (TT3) levels in rats were studied. Furthermore, it was investigated whether the observed effects were related to the serum levels of HCB or PCP. Rats received either corn oil (controls) or HCB, PCP or TCHQ in a single equimolar intraperitoneal dose of 0.056 mmol/kg. Results indicated that HCB did not alter serum TT4 and TT3 levels for a period up to 96 h after dosing. In contrast, PCP and TCHQ were both capable of reducing serum TT4 levels with a maximum effect between 6 and 24 h after exposure. TCHQ was more effective in repressing TT3 than TT4 blood levels. Dose-response experiments were carried out in order to obtain insight into the sensitivity of the observed effects. Rats received different doses of PCP or TCHQ intraperitoneally. The reductions of TT4 levels by PCP were inversely related to serum PCP levels in exposed animals, based on the toxicokinetics and dose-response profiles. Furthermore, PCP serum levels after HCB administration appeared too low to cause an effect. The results of this study indicate that not HCB itself, but rather its metabolites PCP and TCHQ may be involved in reduced serum thyroid hormone levels after HCB administration.

Liver ferrochelatase from normal and hexachlorobenzene porphyric rats. Mechanism of drug action

1. The action of hexachlorobenzene (HCB) on hepatic ferrochelatase was investigated. 2. A direct action of HCB, pentachlorophenol, porphyrins and haem on this enzyme activity was discarded. 3. In HCB porphyric liver there is probably an activator tightly bound to the enzyme. 4. Pyridoxal phosphate (PPL) may be a cofactor of ferrochelatase from both normal and porphyric rats. 5. The PPL would be involved in the binding site of Fe2+ or at least in the approaching of Fe2+ to the active site of the enzyme. 6. The differences found between normal and porphyric preparations could be attributed to conformational changes elicited by the HCB.

Studies on the active centre of rat liver porphyrinogen carboxylase in vivo effect of hexachlorobenzene

1. Porphyrinogen carboxylase from the liver of normal and hexachlorobenzene porphyric rats was subjected to chemical modification using photo-oxidation with methylene blue, diethylpyrocarbonate, butane-2,3-dione, and phenylglyoxal. 2. All of these chemicals inactivated the enzyme from both sources. 3. Reversion of the diethylpyrocarbonate reaction with hydroxylamine as well as protection of the enzymes with uroporphyrinogen III indicated that histidine is involved at least in the first decarboxylation active site of the porphyrinogen carboxylyase, and perhaps in one or more sites where the removal of the other carboxyl groups take place. 4. Arginine seems not to be at the active site of the enzyme but at its environment since two diketones alter the enzyme activity, however the substrate did not protect the enzyme from the butane-2,3-dione modification. 5. Comparative studies between the enzyme from normal and porphyric animals suggest that the low enzyme activity from intoxicated animals could be due to alterations of its active centre environment produced by hexachlorobenzene treatment. This treatment seems to partially protect the active site of the porphyrinogen carboxylase from the modification reactions.

Synergistic effect of mammary tumors on hexachlorobenzene-induced porphyria in rats

The response of animals bearing N-nitroso-N-methylurea (NMU)-induced mammary tumors to the porphyrinogenic action of hexachlorobenzene (HCB) was studied. delta-Aminolevulinic acid (ALA), porphobilinogen and porphyrins in urine, ALA-synthase and porphyrinogen carboxylase activities and porphyrin content in liver and tumor were measured. The results obtained indicate that the metabolic heme pathway operates in mammary tumors but tumor response to HCB treatment could not be detected. HCB administration produced an earlier and greater hepatic porphyria in tumor-bearing rats than in healthy rats suggesting that the presence of tumors exacerbates the action of HCB.

Dose-response relationships in hexachlorobenzene-induced porphyria

The rate of development of hexachlorobenzene (HCB)-induced porphyria in female Wistar rats was determined using HCB dosage and porphyrin analysis protocols designed to determine factors which contribute to the delay commonly observed between initial exposure to HCB and the detection of porphyria. Measurements were made of HCB and porphyrin concentrations in the livers, kidneys, and spleens of female Wistar rats exposed continuously (up to 56 days) or for 1 day to HCB (at dietary concentrations of 1000 ppm and 100 ppm). The experiments showed that when a corn oil solution of HCB was added to the diet at a concentration of 1000 ppm, HCB accumulated rapidly in all organs, and the delay in appearance of elevated liver highly carboxylated porphyrins (HCPs) was at most 4 days (approximately 8-fold elevation of HCPs on day 4). One day of exposure to this diet was sufficient to cause elevated liver HCPs, thus showing that continuous exposure to HCB was not required to cause porphyria in this species. Solid HCB added directly to the diet (1000 ppm) resulted in less rapid HCB accumulation and less rapid development of porphyria. The experiments demonstrated that the appearance of a delay in HCB-induced porphyria in the Wistar rat is caused by the rate at which HCB is absorbed, and by using total hepatic porphyrins (rather than HCPs) as the indicator of the disorder. The experiments also showed that HCB-induced liver enlargement and neurotoxicity are not necessarily associated with the severity of porphyria.

Thyroid function and thyroxine metabolism in hexachlorobenzene-induced porphyria

The effects of hexachlorobenzene (HCB) administration on the development of porphyria and on changes in thyroid function and thyroid hormone metabolism were examined. Female Wistar rats were treated with HCB for 1 or 8 weeks. At both treatment times liver weight was notably increased with a slight change in thyroid weight at 8 weeks. Serum thyroxine (T4) levels were depressed, whereas levels of triiodothyronine (T3) were not depressed significantly at both treatment times. One or eight weeks of HCB treatment did not alter the incorporation and distribution of [125I] into intrathyroidal aminoacids. A 50% reduction in protein bound iodine (PB[125I]) was seen in both groups of animals. HCB altered [125I]T4 metabolism in rat liver slices, increasing T4 dehalogenation. HCB administration for 1 week did not affect urinary excretion of porphyrins or their precursors, or hepatic porphyrin content. The activity of aminolaevulinate synthase was not affected, but there was a 25% and 51% inhibition in porphyrinogen carboxy-lyase (PCL) activity for the uroporphyrinogen disappearance or the coproporphyrinogen formation respectively. After 8 weeks of HCB administration the rats showed a characteristic porphyria. Our results show that HCB treatment increased hepatic thyroxine metabolism, without alterations in thyroid hormone synthesis. Serum T4 and PCL activity behaved differently in both time- and dose-dependent studies, with serum T4 being the more sensitive parameter which responded at earlier times and lower doses.

The relation between the oxidative biotransformation of hexachlorobenzene and its porphyrinogenic activity

The relation between the major toxic effect of hexachlorobenzene, hepatic porphyria, and its oxidative biotransformation was studied in vivo, by observing the effect of modulating its biotransformation on the expression of porphyria. This modulation was achieved by selective in vivo inhibition of the major cytochrome P450 isoenzyme involved in both the hydroxylation of hexachlorobenzene and its primary oxidative metabolite, pentachlorophenol. The involvement of this isoenzyme, cytochrome P450p, was established by in vitro biotransformation studies using microsomes derived from rats treated with various inducers of cytochrome P450 isoenzymes and selective in vitro inactivation of cytochrome P450p by triacetyloleandomycin (TAO), resulting in a strong inhibition of the microsomal conversion of hexachlorobenzene and pentachlorophenol. In vivo inactivation of cytochrome P450p was achieved by coadministration of hexachlorobenzene and TAO. Female rats which were treated with this diet for 10 weeks showed a strongly diminished urinary excretion of the major oxidative metabolites, pentachlorophenol and tetrachloro-1,4-hydroquinone, as compared to rats treated with hexachlorobenzene alone. The TAO coadministration was found to result in complexation of 70% of the total amount of hepatic microsomal cytochrome P450. The group treated with hexachlorobenzene alone displayed a 600-fold increase in the amount of hepatic porphyrins, whereas an almost complete absence of hepatic porphyrins was observed after administration of hexachlorobenzene together with TAO. The urinary excretion of porphyrins was also significantly lowered by cotreatment with TAO. A strong correlation was found to exist between the amount of porphyrins excreted and the amount of oxidative metabolites excreted, as a function of exposure time. Glucuronidation of pentachlorophenol was observed to an average extent of 30%. This percentage was not influenced by either TAO or phenobarbital. These results suggest that oxidative biotransformation, and thus the formation of the very reactive tetrachloro-1,4-benzoquinone, is directly related to the porphyrinogenic action of hexachlorobenzene.

Development of hexachlorobenzene porphyria in rats: time sequence and relationship with lipid peroxidation

The relationship between the development of porphyria and free-radical formation induced by hexachlorobenzene was studied in iron-overloaded rats. The first sign of porphyria, an increase in porphyrins in the liver, was detected at day 22. Liver malondialdehyde was also increased at day 22. During the following weeks, liver porphyrins and malondialdehyde increased simultaneously, accompanied by a decrease in uroporphyrinogen decarboxylase activity and glucose-6-phosphate activity in liver, and a high excretion of porphyrins in the urine. In the rats given hexachlorobenzene, changes were detected in the pattern of lipids in the liver microsomes. In comparison with the controls, there were decreases in C20:4 and C22:5 fatty acids, whereas the fatty acid C20:3w6 was increased. In this study of hexachlorobenzene-induced liver damage there was no difference in the time course of the development of porphyria and that of lipid peroxidation.

Possible reactive intermediates in the oxidative biotransformation of hexachlorobenzene

In this review the biotransformation of hexachlorobenzene is discussed, with special reference to the possible generation of reactive metabolites or intermediates during this process. Evidence is presented for the direct involvement of certain cytochrome P-450 isoenzymes in the major toxic effect of hexachlorobenzene, hepatic porphyria. The in vivo biotransformation is discussed and compared with in vitro experiments (microsomal and cell culture studies). The possible reactive metabolites and intermediates and their mechanisms of formation are presented. Special attention is directed to a very reactive metabolite, tetrachloro-1,4-benzoquinone, which has a high capacity to efficiently react with protein, thus possibly linking the oxidative biotransformation of hexachlorobenzene and the molecular mechanism of enzyme inactivation leading to hepatic porphyria.

Influence of the strain of rats on the induction of hexachlorobenzene induced porphyria

1. The present work undertakes a comparative study on the hexachlorobenzene (HCB) porphyria induction in female rats of Wistar and CHBBTHOM strains. The purpose was to characterize the CHBBTHOM strain with respect to the haem metabolic pathway, its regulatory mechanisms and its response to foreign drugs. 2. After 7 weeks of treatment it was observed that the hepatic porphyrins increased 140 times, ALA-synthase 4 times and PCL was 73% inhibited in the Wistar strain. 3. On the other hand the animals of CHBBTHOM strain showed lesser alteration on these parameters; hepatic porphyrins increased only 3-fold, ALA-synthase 1.7-fold and PLC was only 22% inhibited. 4. Total iron liver content was nearly equal in both strains of rats. 5. The results obtained would indicate that the lower susceptibility of the CHBBTHOM strain to acquire porphyria does not seem to be due to either: (1) congenital alterations of any parameters of the haem metabolic pathway, since the behaviour of normal animals from both strains was similar; or (2) a lower hepatic iron content in such animals. 6. These findings would suggest that the differential response to HCB to this strain would be looked for in another metabolic pathway, such as that involved in the metabolization process of the toxic.

Liver ferrochelatase from normal and hexachlorobenzene porphyric rats. Studies on their properties

1. The aim of the present work is to shed light on the way of action of hexachlorobenzene (HCB) on hepatic ferrochelatase the mitochondrial enzyme which catalyzes the last step of haem biosynthetic pathway. 2. Some properties of this enzyme from normal and HCB porphyric rat liver were studied. 3. The present findings indicate that HCB treatment would modify the configuration of the enzyme perhaps allowing the active center of the porphyric ferrochelatase to be more exposed. 4. As a consequence it would show: (a) its higher affinity for the iron; (b) the shorter time necessary to form the intermediate enzyme-substrate, reflected both by the existence of a shorter lag and consequently a shorter pre incubation time. 5. However this modification elicited by the fungicide does not alter the submitochondrial distribution of the enzyme nor the optimal conditions for its measurement.

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

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.

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

Exposure of rats to HCB caused a dose-dependent depletion of GSH. Chlorophenolic and sulfur-containing metabolites of HCB incubated with GSH-free rat liver cytosolic protein drastically diminished the UROD activity. In addition, HCB also exhibited inhibitory potency. The most effective compounds studied were TCH and its oxidation product, chloranil. Incubation of liver cytosolic protein and of GSH with HCB and its metabolites yielded results that suggested interaction between the compounds and cell constituents--an interaction that may cause inhibition of the hepatic UROD activity in the HCB-exposed organism.

Role of inhibition of uroporphyrinogen decarboxylase in PCB-induced porphyria in mice

The oral administration of 3,4,5,3',4',5'-hexachlorobiphenyl for 3 weeks to mice caused a marked accumulation of porphyrins in the liver of C57BL/6 and C57Bl/10 mice but not in the liver of ddY mice. The time course of induction of delta-aminolevulinic acid synthetase (ALA-S), cytochrome P-450, and mixed function oxidases and inhibition of uroporphyrinogen decarboxylase (URO-D) in the liver of C57BL/6 mice and ddY mice fed a diet containing 500 ppm of a commercial PCB (Kanechlor-500) were investigated to clarify the sole factor in inducing porphyria. The activity of URO-D in the liver of C57BL/6 mice was depressed approximately 80% at 3 weeks when a large amount of uroporphyrin accumulated. Male ddY mice showed only a slight increase in uroporphyrin accumulation in the liver and a moderate decrease of URO-D activity even at the 10th week. ALA-S, cytochrome P-450, and mixed function oxidases were induced in both strains of mice, although the magnitude of these inductions in C57BL/6 mice was greater than that in ddY mice. No differences were detected between the two strains in the content and gas chromatographic pattern of PCB remaining in liver cytosol (6 weeks). In addition there was no relationship between the time of onset of porphyria and that of the maximal induction of drug-metabolizing function in C57BL/6 mice. These results indicate that the development of porphyria is causally related to the inhibition of URO-D rather than the induction of drug-metabolizing function. The hypothesis that porphyria first develops when the ratio of hepatic URO-D and ALA-S activities decreases to less than 1.0 is presented.

Liver porphyrinogen carboxylase in hexachlorobenzene porphyric rats. Studies with intermediate porphyrinogens of series III and with uroporphyrinogen I

The present work studies the action of hexachlorobenzene (HCB) on the decarboxylation of uroporphyrinogen (Urogen) I and III and also on the decarboxylation of intermediate porphyrinogens of series III under different conditions using liver of normal and porphyric rats as enzyme source. The same enzyme is involved in the Urogen decarboxylation of both isomeric series I and III and catalyses the four steps in both cases. HCB affects all of them. HCB blocks the four steps of Urogen III decarboxylation to the same degree, as a function of intoxication time. HCB leads, in general, to an increase in the efficiency (Km/Vmax) of the porphyric system. These data can be interpreted as a reaction of the organism to overcome the enzymatic blockade.

The microsomal metabolism of hexachlorobenzene. Origin of the covalent binding to protein

The microsomal metabolism of hexachlorobenzene is studied, with special attention to the covalent binding to protein. The metabolites formed are pentachlorophenol and tetrachlorohydroquinone. In addition, a considerable amount of covalent binding to protein is detected (250 pmoles pentachlorophenol, 17 pmoles tetrachlorohydroquinone and 11 pmoles covalent binding in an incubation containing 50 mumoles of hexachlorobenzene). In order to establish the potential role of reductive dechlorination in the covalent binding, the anaerobic metabolism of hexachlorobenzene was investigated. At low oxygen concentrations no pentachlorobenzene was detected, and only very small amounts of pentachlorophenol as well as covalent binding, indicating a relationship between covalent binding and the microsomal oxidation of hexachlorobenzene. Incubations with 14C-pentachlorophenol at low concentrations showed that a conversion-dependent covalent binding occurs to the extent of 75 pmole binding per nmole pentachlorophenol. This is almost enough to account for the amount of label bound to protein observed in hexachlorobenzene incubations. This indicates that less than 10% of the covalent binding occurs during conversion of hexachlorobenzene to pentachlorophenol, and the remainder is produced during conversion of hexachlorobenzene to pentachlorophenol, and the remainder is produced during conversion of pentachlorophenol. The major product of microsomal oxidation of pentachlorophenol is tetrachlorohydroquinone, which is in redox-equilibrium with the corresponding semiquinone and quinone (chloranil). The covalent binding is inhibited by addition of ascorbic acid or glutathione to the hexachlorobenzene incubations. Ascorbic acid decreases the covalent binding with a simultaneous increase in formation of tetrachlorohydroquinone, probably due to a shift in the redox-equilibrium to the reduced side. Glutathione does not act as a reducing agent, since the inhibition of covalent binding is not accompanied by an increase in tetrachlorohydroquinone formation. Instead, glutathione reacts with chloranil, producing at least three stable products, probably in a Michael-type reaction. These results strongly indicate the involvement of chloranil or the semiquinone radical in the covalent binding during microsomal hexachlorobenzene metabolism.

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

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.

Tissue porphyrin pattern determination by high-speed high-performance liquid chromatography

A rapid and sensitive method for the determination of porphyrin carboxylic acids in liver, kidney, and spleen by high-speed high-performance liquid chromatography is described. Porphyrins were extracted with recoveries greater than or equal to 98%, concentrated on disposable octadecylsilyl cartridges, and analyzed with a liquid chromatograph equipped with a 3 microns X 3 cm octadecylsilyl column and a fluorescence detector. Separation of di-, tetra-, penta-, hexa-, hepta-, and octacarboxylic acids was achieved within 5 min. The detection limits for uro, copro, and protoporphyrin were 20, 10, and 20 fmol, respectively.

Effect of desferrioxamine on the development of hexachlorobenzene-induced porphyria

The present work deals with the effect of desferrioxamine (DF) on hexachlorobenzene (HCB)-induced porphyria in female rats with the purpose of further investigation of the role of iron in the development of this porphyria. The results obtained show that the repeated injection of DF (three times a week: 100 mg/kg each i.m.) delayed and diminished remarkably the urinary excretion of precursors and porphyrins as well as the accumulation of the latter in liver promoted by HCB (1 g/kg daily given by stomach tube). This was probably due to attenuation by DF of the alterations produced by the fungicide in the two key enzymes: porphyrinogen carboxy-lyase (PCL) and delta-aminolaevulinate synthase (ALA-S). In fact, DF by reducing liver iron levels produced a smaller decrease of the target enzyme (PCL) and a concomitant smaller induction of ALA-S. DF alone did not modify any of these variables or the liver to body weight ratio. DF added at 10(-2) and 10(-3) M to the incubation media of ALA-S and PCL did not alter either of the enzymatic activities, whether in normal or HCB-porphyric preparations. The results obtained show that DF improved the biochemical picture during HCB porphyria. They suggest that iron plays an indirect role in the decrease of PCL enzyme, possibly at the HCB metabolization step. A common iron-involving mechanism for the production of porphyria by different chlorinated compounds is suggested.

Uroporphyrin accumulation produced by halogenated biphenyls in chick-embryo hepatocytes. Reversal of the accumulation by piperonyl butoxide

Cultures of chick-embryo hepatocytes were used to study the mechanism by which 3,4,3',4'-tetrachlorobiphenyl and 2,4,5,3',4'-pentabromobiphenyl cause accumulation of uroporphyrin. In a previous paper, an isoenzyme of cytochrome P-450 induced by 3-methylcholanthrene had been implicated in this process [Sinclair, Bement, Bonkovsky & Sinclair (1984) Biochem. J. 222, 737-748]. Cells treated with 3,4,3',4'-tetrachlorobiphenyl and 5-aminolaevulinate accumulated uroporphyrin and heptacarboxyporphyrin, whereas similarly treated cells accumulated protoporphyrin immediately after piperonyl butoxide was added. Piperonyl butoxide also restored haem synthesis as detected by incorporation of radioactive 5-aminolaevulinate into haem, and decrease in drug-induced 5-aminolaevulinate synthase activity. The restoration of synthesis of protoporphyrin and haem by piperonyl butoxide was not affected by addition of cycloheximide, indicating recovery was probably not due to protein synthesis de novo. Piperonyl butoxide also reversed uroporphyrin accumulation caused by 3,4,5,3',4',5'-hexachlorobiphenyl, mixtures of other halogenated biphenyls, lindane, parathion, nifedipine and verapamil. The effect of piperonyl butoxide was probably not due to inhibition of metabolism of these compounds, since the hexachlorobiphenyl was scarcely metabolized. Other methylenedioxyphenyl compounds, as well as ellipticine and acetylaminofluorene, also reversed the uroporphyrin accumulation caused by 3,4,3',4'-tetrachlorobiphenyl. SKF-525A (2-dimethylaminoethyl-2,2-diphenyl valerate) did not reverse the uroporphyrin accumulation caused by the halogenated biphenyls, but did reverse that caused by phenobarbital and propylisopropylacetamide. We conclude that the mechanism of the uroporphyrin accumulation cannot be due to covalent binding of activated metabolites of halogenated compounds to uroporphyrinogen decarboxylase.

Influence of chemical modification on enzyme inactivation kinetics and stability

Enzymes are placed in different categories depending on the effect of chemical modification on their inactivation kinetics and residual activity. This is done using a series-type mechanism involving degraded but stable enzyme states. The major distinction in the three basic categories is the effect of modification on residual activity. Each category is further sub-divided depending on the effect of modification on the values of the deactivation rate constants. The classification provides for a framework for comparison of a wide variety of enzyme deactivation data. Structure-function relations are provided wherever possible.

A mathematical analysis of aging influences on enzyme deactivation/activation kinetics. Examples of the influence of regional brain development and drugs in rats

A series-type enzyme deactivation/activation model involving active enzyme states is utilized to theoretically quantify the influence of regional brain development and drugs on enzyme activity levels in rats. Continuous hexachlorobenzene administration with, or without, phenobarbitone pretreatment has different effects on the deactivation/activation kinetics of porphyrinogen carboxylase, delta-aminolaevulinate synthase and delta-aminolaevulinate dehydratase. The deactivation/activation kinetics exhibited by pyruvate dehydrogenase, citrate synthase, and D-3-hydroxybutyrate dehydrogenase during the development of the medulla oblongata, mid-brain, striatum, and hypothalamus sections exhibit similarities as well as discrepancies. These are identified and made more quantitative.

Formation of pentachlorophenol as the major product of microsomal oxidation of hexachlorobenzene

On incubation of [14C]-hexachlorobenzene with microsomes from livers of rats induced with hexachlorobenzene, the major product (80-90%) was pentachlorophenol. The only other detectable metabolite, tetrachlorohydroquinone (4-15%), was presumably formed from pentachlorophenol. A considerable amount of radioactivity (5-10% of the amount of extracted metabolites) was covalently bound to protein. Microsomes derived from male hexachlorobenzene--induced rats gave by far the highest conversion (approx. 1% of substrate). Microsomes from female hexachlorobenzene--induced rats were 3 times less efficient. Microsomes from untreated and 3-methyl-cholanthrene--treated animals gave less than 5% of the amount of pentachlorophenol formed by microsomes from hexachlorobenzene--induced male rats, while phenobarbital and aroclor 1254-induction resulted in formation of 51% and 34% respectively.