A sulphite-oxidase-deficient rat model: metabolic characterization

The effect of ingested sulfite on active avoidance in normal and sulfite oxidase-deficient aged rats

The aim of this study was to investigate the possible toxic effects of sulfite on neurons by measuring active avoidance learning in normal and sulfite oxidase (SOX)-deficient aged rats. Twenty-four months of age Wistar rats were divided into four groups: control (C), sulfite-treated group (S), SOX-deficient group (D) and SOX-deficient + sulfite-treated group (DS). SOX deficiency was established by feeding rats with a low molybdenum (Mo) diet and adding 200 ppm tungsten (W) to their drinking water. Sulfite in the form of sodium metabisulfite (25 mg/kg) was given by gavage for six weeks. Active avoidance responses were determined by using an automated shuttle box. Hepatic SOX activity was measured to confirm SOX deficiency. The hippocampus was used for determining the activity of cyclooxygenase (COX) and caspase-3 enzymes and the level of prostaglandin E2 (PGE2) and nitrate/nitrite. SOX-deficient rats had an approximately 10-fold decrease in hepatic SOX activity compared with normal rats. Sulfite did not induce impairment of active avoidance learning in SOX-deficient rats and in normal rats compared with their control groups. Sulfite had no effect on the activity of COX and caspase-3 in the hippocampus. Treatment with sulfite did not significantly increase the level of PGE2 and nitrate/nitrite in the hippocampus.

Mechanism of sulfite cytotoxicity in isolated rat hepatocytes

Sulfite (SO(3)(2-)) has been widely used as preservative and antimicrobial in preventing browning of foods and beverages. SO(2), a common air pollutant, also is capable of producing sulfite and bisulfite depending on the pH of solutions. A molybdenum-dependent mitochondrial enzyme, sulfite oxidase, oxidizes sulfite to inorganic sulfate and prevents its toxic effects. In the present study, sulfite toxicity towards isolated rat hepatocytes was markedly increased by partial inhibition of cytochrome a/a(3) by cyanide or by putting rats on a high-tungsten/low-molybdenum diet, which result in inactivation of sulfite oxidase. Sulfite cytotoxicity was accompanied by a rapid disappearance of GSSG followed by a slow depletion of reduced glutathione (GSH). Depleting hepatocyte GSH beforehand increased cytotoxicity of sulfite. On the other hand, dithiothreitol (DTT), a thiol reductant, added even 1h after the addition of sulfite to hepatocytes, prevented cell death and restored hepatocyte GSH levels. Sulfite cytotoxicity was also accompanied by an increase of oxygen uptake, reactive oxygen species (ROS) formation and lipid peroxidation. Cytochrome P450 inhibitors, metyrapone and piperonyl butoxide also prevented sulfite-induced cytotoxicity and lipid peroxidation. Desferroxamine and antioxidants also protected the cells against sulfite toxicity. These findings suggest that cytotoxicity of sulfite is mediated by free radicals as ROS formation increases by sulfite and antioxidants prevent its toxicity. Reaction of sulfite or its free radical metabolite with disulfide bonds of GSSG and GSH results in the compromise of GSH/GSSG antioxidant system leaving the cell susceptible to oxidative stress. Restoring GSH content of the cell or protein-SH groups by DTT can prevent sulfite cytotoxicity.

Effect of sulfite on macrophage functions of normal and sulfite oxidase-deficient rats

Sulfite has both an endogenous and an exogenous provenance in the mammalian tissues. The aim of the present study was to assess the effect of sulfite on macrophages functions in normal or sulfite oxidase deficient rats. Rats were divided into eight groups; (1) control group, (2) sulfite group (the rats received sodium meta bi-sulfite (25 mg/kg) in drinking water for 6 weeks), (3) vitamin E group (the rats received Vit E (50 mg/kg) by gavage for 6 weeks), (4) sulfite group+Vit E, (5)sulfite oxidase deficient group (the rats received high-W/Mo-deficient diet. The activity of sulfite oxidase was reduced in rats maintained on the high-W/Mo-deficient diet during the first 21 days of treatment. After the sulfite-oxidase deficiency, the rats continued to receive high-W/Mo-deficient diet for 6 weeks.), (6) sulfite+sulfite oxidase deficient group, (7) Vit E+sulfite oxidase deficient group, and (8) sulfite+Vit E+sulfite oxidase deficient group. Sulfite caused a significant increase in phagocytic and chemotactic activities of peritoneal macrophages. In sulfite-oxidase deficient rats, the increase in phagocytic and chemotactic activities in peritoneal macrophages after sulfite intake was found more than the control rats. Vit E supplementation prevented sulfite induced increase in macrophages functions. These results show that the macrophage functions are sensitive to sulfite intake. The effect of sulfite on macrophage functions may be related to reactive oxygen species. Because Vit E administration was able to modulate significantly sulfite-induced changes in the functions of peritoneal macrophages.

Molybdenum: an essential trace element

Molybdenum is found in most foods, with legumes, dairy products, and meats being the richest sources. This metal is considered essential because it is part of a complex called molybdenum cofactor that is required for the three mammalian enzymes xanthine oxidase (XO), aldehyde oxidase (AO), and sulfite oxidase (SO). XO participates in the metabolism of purines, AO catalyzes the conversion of aldehydes to acids, and SO is involved in the metabolism of sulfur-containing amino acids. Molybdenum deficiency is not found in free-living humans, but deficiency is reported in a patient receiving prolonged total parenteral nutrition with clinical signs characterized by tachycardia, headache, mental disturbances, and coma. The biochemical abnormalities in this acquired molybdenum deficiency include very low levels of uric acid in serum and urine (low XO activity) and low inorganic sulfate levels in urine (low SO activity). Inborn errors of isolated deficiencies of XO, SO, and molybdenum cofactor are described. Although XO deficiency is relatively benign, patients with isolated deficiencies of SO or molybdenum cofactor exhibit mental retardation, neurologic problems, and ocular lens dislocation. These abnormalities seem to be caused by the toxicity of sulfite and/or inadequate amounts of inorganic sulfate available for the formation of sulfated compounds present in the brain. XO and AO may also participate in the inactivation of some toxic substances, inasmuch as studies suggest that molybdenum deficiency is a factor in the higher incidence of esophageal cancer in populations consuming food grown in molybdenum-poor soil.

Toxicology of sulphiting agents. I: Animal studies

Oral toxicity studies of sulphite in animals are briefly reviewed. On the basis of a chronic toxicity/carcinogenicity study of sodium metabisulphite in rats JECFA established an ADI of 0.7 mg SO2/kg using a safety factor of 100. The effects of sodium metabisulphite (1% Na2S2O5) on the composition of semi-purified rat diets during storage for at least three months have been studied. Under these conditions sulphite induced rancid off-flavours in diets containing unsaturated fats. The feeding of such stored sulphited-diets to rats resulted in growth retardation and diminished food efficiency. The extractability of the lipids from these diets was markedly reduced. Extraction with hexane, followed by ethanol extraction did not remove the toxic factor(s). It was suggested that sulphite in stored semi-purified diets may react with unsaturated fats leading to the formation of polymers of unsaturated fatty acids and/or other toxic substances. It is recommended that special attention is paid to the fatty acid composition of sulphited foods to be stored.

Trace minerals in the nutrition of children

Trace elements perform important functions in growth and development. However, little information exists about their dietary requirements during the demanding period of infancy. Opportunities to add to knowledge of the physiologic significance and dietary adequacy of trace elements in human nutrition are provided by recent analytic advances. Specific, sensitive, and reliable methods for the detection of trace element imbalances are sorely needed. Although several factors influence the dietary needs of these essential elements, the basis for establishing dietary needs in infants is hindered by the death of studies that have assessed their bioavailability in this age group. Thus until it has been conclusively shown otherwise, the physiologic response to human milk is used as the standard for infant feeding practices. This review is limited primarily to the physiologic significance and bioavailability of zinc, copper, manganese, molybdenum, chromium, fluoride, and selenium. The space devoted to each trace element is not meant to represent the element's importance but, rather, to reflect some of the present understanding of its metabolism and utilization.

Comparative subchronic oral toxicity of sulphite and acetaldehyde hydroxysulphonate in rats

A subchronic oral toxicity study of sodium metabisulphite and acetaldehyde hydroxysulphonate was conducted in normal and sulphite oxidase-deficient rats. At the highest dose level (350 mg SO2 equiv./kg body weight/day for 3 wk followed by 175 mg SO2 equiv./kg body weight/day for 5 wk of either compound), gastric lesions were noted histologically in both normal and sulphite oxidase-deficient rats. The lesions were more severe and more frequently encountered in the sulphite oxidase-deficient rats. The no-effect level for Na2S2O5 was 70 mg SO2 equiv./kg body weight/day in both normal and sulphite oxidase-deficient rats. Liver lesions were noted in rats treated with acetaldehyde hydroxysulphonate. These lesions may possibly be attributable to the effects of free acetaldehyde. The no-effect level for acetaldehyde hydroxysulphonate was 7 mg SO2 equiv./kg body weight/day for sulphite oxidase-deficient rats and 70 mg SO2 equiv./kg body weight/day for normal rats. Increased urinary excretion of sulphite was noted in sulphite oxidase-deficient rats whether or not they were given exogenous sulphites. An increase in urinary sulphite levels in sulphite oxidase-deficient rats was observed after dosing with acetaldehyde hydroxy-sulphonate. These findings suggest that acetaldehyde hydroxysulphonate is metabolized to acetaldehyde and free sulphite.

Distribution, metabolism and toxicity of inhaled sulfur dioxide and endogenously generated sulfite in the respiratory tract of normal and sulfite oxidase-deficient rats

We report on the distribution, metabolism, and toxicity of sulfite in the respiratory tract and other tissues of rats exposed to endogenously generated sulfite or to inhaled sulfur dioxide (SO2). Graded sulfite oxidase deficiency was induced in several groups of rats by manipulating their tungsten to molybdenum intake ratio. Endogenously generated sulfite and S-sulfonate compounds (a class of sulfite metabolite) accumulated in the respiratory tract tissues and in the plasma of these rats in inverse proportion to hepatic sulfite oxidase activity. In contrast to this systemic mode of exposure, sulfite exposure of normal, sulfite oxidase-competent rats via inhaled SO2 (10 and 30 ppm) was restricted to the airways. Minor pathological changes consisting of epithelial hyperplasia, mucoid degeneration, and desquamation of epithelium were observed only in the tracheas and bronchi of the rats inhaling SO2, even though the concentration of sulfite plus S-sulfonates in the tracheas and bronchi of these rats was considerably lower than that in the endogenously exposed rats. We attribute this histological damage to hydrogen ions stemming from inhaled SO2, not to the sulfite/bisulfite ions that are also a product of inhaled SO2. In addition to the lungs and trachea, all other tissues examined, except the testes, appeared to be refractory to high concentrations of endogenously generated sulfite. The testes of grossly sulfite oxidase-deficient rats were severely atrophied and devoid of spermatogenic cells.

Sulfite hypersensitivity. A critical review

Sulfiting agents (sulfur dioxide and the sodium and potassium salts of bisulfite, sulfite, and metabisulfite) are widely used as preservatives in foods, beverages, and pharmaceuticals. Within the past 5 years, there have been numerous reports of adverse reactions to sulfiting agents. This review presents a comprehensive compilation and discussion of reports describing reactions to ingested, inhaled, and parenterally administered sulfite. Sulfite hypersensitivity is usually, but not exclusively, found within the chronic asthmatic population. Although there is some disagreement on its prevalence, a number of studies have indicated that 5 to 10% of all chronic asthmatics are sulfite hypersensitive. This review also describes respiratory sulfur dioxide sensitivity which essentially all asthmatics experience. Possible mechanisms of sulfite hypersensitivity and sulfur dioxide sensitivity are discussed in detail. Sulfite metabolism and the role of sulfite oxidase in the detoxification of exogenous sulfite are reviewed in relationship to the etiology of sulfite hypersensitivity.

[Sulfites in foods--a health risk?]

This short review summarizes properties, applications and effects of sulfite in food products, including sulfite residues in foods produced according to legal regulations. Sulfite metabolism and toxicology in the mammalian organism as well as appearance of adverse reactions following ingestion of sulfite-treated food are discussed. Although knowledge in this area is still incomplete, the continued use of sulfite in food technology can be justified.

A possible involvement of glutathione in the detoxication of sulfite

Inorganic sulfite may be detoxified by conversion to S-sulfocysteine. We demonstrate this conversion by a series of enzyme-catalyzed steps as follows. Inorganic sulfite reacts with glutathione disulfide by a thiol transferase catalyzed reaction as previously demonstrated. The S-sulfoglutathione formed is then converted by gamma-glutamyltranspeptidase to S-sulfocysteinylglycine and the latter finally hydrolyzed to S-sulfocysteine by a renal dipeptidase. S-Sulfoglutathione is a substrate for gamma-glutamyl-transpeptidase as effective as glutathione itself. Furthermore, S-sulfocysteinylglycine is cleaved as efficiently as cysteinylglycine by a renal dipeptidase at high substrate concentrations but somewhat less efficiently at low substrate concentrations.

Sulphiting agents in foods: some risk/benefit considerations

The current toxicological status of sulphiting agents is reviewed, including evidence of adverse reactions to sulphited foods by a sub-population of asthmatics. Against this background are assessed the applications and benefits of sulphiting agents in foods. It is concluded that further information is required to determine the magnitude of risk and that, in the interim, the controlled use of sulphiting agents is justifiable.

A sulphite oxidase-deficient rat model: reproductive toxicology of sulphite in the female

The reproductive toxicology of sulphite was investigated in female rats with induced deficiencies of sulphite oxidase. Induction of sulphite oxidase deficiency was accomplished by administration of a diet with a high tungsten to molybdenum ratio. This animal model was chosen because it enables exposure of tissues to high systemic sulphite concentrations without debilitating side effects. The reproductive performance of female sulphite oxidase-deficient rats, exposed continuously to sulphite from about 3 wk before mating until the experiment was terminated on day 20 of gestation, was compared to that of unexposed rats with normal sulphite oxidase activity. There was no treatment-related trend in any of the parameters observed, including mating and pregnancy rates, gestational weight gain, pre-implantation loss, resorbed and dead foetuses, litter size, foetal weight and malformations. Of the spectrum of malformations observed in control and treated animals, only anophthalmia may have been treatment related. From these experiments, performed in an animal model that is a conservative metabolic analogue for man, we have concluded that there is no evidence to indicate that exposure of females to sulphite poses a significant reproductive hazard.

Identification of plasma proteins containing sulfite-reactive disulfide bonds

Plasma protein S-sulfonate compounds (RS-SO-3) have previously been shown to form, presumably by sulfitolysis of disulfide bonds, as a result of exposure to sulfite. In the investigations reported here, we identify two proteins in rabbit plasma, namely albumin and plasma fibronectin, which contain reactive sites for S-sulfonate formation. Separation and identification of these proteins following in vitro and in vivo exposure to sulfite was accomplished primarily by column chromatographic and electrophoretic techniques. In addition, the structure of presumed S-sulfonate groups was confirmed by the identification of cysteinyl-S-sulfonate residues in protein hydrolysates generated by enzymatic digestion. The molar ratio of RS-SO-3 in both albumin and plasma fibronectin was less than one. Data from our experiments suggest that the mixed disulfide site of non- mercaptalbumin is the reactive site for S-sulfonate formation. The site(s) of formation within the plasma fibronectin molecule was not investigated. The possible physiological significance of disulfide sulfitolysis of albumin and plasma fibronectin is discussed.

Reversed-phase ion-pair liquid chromatographic procedure with electrochemical detection for the analysis of urinary thiosulphate

A method using high-performance liquid chromatography with mercury-based electrochemical detection has been developed for the determination of thiosulphate in urine. The chromatographic separation is based upon ion-pair formation between thiosulphate and tetrabutylammonium and reversed-phase chromatography. The method was compared with an earlier reported colorimetric assay and found to be superior with respect to specificity and sensitivity.

Attempts to induce cytogenetic effects with sulphite in sulphite oxidase-deficient Chinese hamsters and mice

Chinese hamsters and mice were made sulphite-oxidase deficient by the feeding of a low-molybdenum diet with sodium tungstate as a drinking-water supplement. Hepatic sulphite-oxidase activity was checked spectrophotometrically. Under normal conditions, sulphite-oxidase activity is high in the mouse and low in the Chinese hamster. Sulphite (SO3--) was given in a single or double oral dose in aqueous solution or dissolved in fruit juice or by repeated subcutaneous injections up to the maximum tolerated doses. Possible cytogenetic effects were studied in bone-marrow cells using three test systems--the sister chromatid exchange, chromosome aberration and micronucleus tests. No induction of cytogenetic effects was observed with any of the three tests in either species, indicating that no damage at the chromosomal level was induced by sulphite in these animals, even when sulphite-oxidase activity was reduced to a very low level.