Tohru Masukawa

Degradation of methylmercury by selenium

When methylmercury was incubated in the presence of selenite and reduced glutathione (GSH), the mercury which was extracted into benzene under acidic condition decreased gradually with the elapse of time. This decrease was due to the cleavage of mercury-carbon bond of methylmercury. The reaction did not proceed when selenite or GSH was singly added to the reaction mixture. L-Cysteine, 2-mercaptoethanol and sodium sulfide in place of GSH also were effective for decomposition of methylmercury in combination with selenite, but oxidized glutathione (GSSG) and L-cystine were not. This suggests that reduction of selenite is needed for the degradation of methylmercury. Thus, the effect of reduced metabolites of selenite produced by GSH was investigated. Glutathione selenotrisulfide (GSSeSG) required GSH for the degradation of methylmercury, whereas H2Se possessed a strong activity even in the absence of GSH. This may indicate that H2Se is involved directly in the conversion of methylmercury to inorganic mercury. This phenomenon found in in vitro experiments is discussed in relation to the biotransformation of methylmercury.

Involvement of tissue sulfhydryls in the formation of a complex of methylmercury with selenium.

Albumin-bound methylmercury was converted to a benzene-extractable form by the soluble fraction of rat liver, kidney or brain in the presence of selenite, but not in its absence. The factors in the soluble fraction causing this conversion were investigated by column chromatography. Sephadex G-25 chromatography showed that effective factors were present in non-protein and protein fractions. It was concluded from ion exchange and Sephadex G-200 chromatography that these factors in the non-protein and protein fractions were reduced glutathione (GSH) and protein sulfhydryl groups respectively. Because GSH and the soluble protein could be replaced by sulfhydryl compounds, such as cysteine and 2-mercaptoethanol, as well as by a purified protein with sulfhydryl groups, reduced ribonuclease (RNase), respectively, it was concluded that sulfhydryl groups of GSH and/or proteins in the soluble fraction were needed for selenite-induced conversion of methylmercury to a benzene-soluble form. Among the various selenium compounds tested, only H2Se (the reduced metabolite of selenite) was found to react directly with methylmercury to form a benzene extractable mercury compound in the absence of the soluble fraction. These findings suggest that the conversion of methylmercury to a benzene-soluble form occurs by reaction of methylmercury with selenium (possibly H2Se) reduced by GSH and/or protein sulfhydryl groups in the soluble fraction. Thin-layer chromatography showed that benzene-extractable mercury consists mainly of bis(methylmercuric) selenide (BMS). A minor component, trismethylmercuric selenonium, was also detected by mass spectrography.

Possible regulation mechanism of microsomal glutathione S-transferase activity in rat liver.

After rats were injected with the reduced glutathione (GSH) depletor phorone (diisopropylidene acetone, 250 mg/kg, i.p.), there was a significant increase in microsomal glutathione S-transferase activity in the liver. The maximum activity was observed 24 hr after injection and was about 2-fold that of the control activity. Diethylmaleate (500 mg/kg, i.p.) had the same effect. Twenty-four hours after phorone injection (250 mg/kg, i.p.), the concentrations of GSH and oxidized glutathione (GSSG) in the liver were increased about 2-fold. Under the same conditions, the level of mixed disulfides with microsomal proteins (GSS-protein) was also increased. Further, the activity of microsomal glutathione S-transferases was increased by the in vitro addition of disulfide compounds such as GSSG, cystine and homocystine, and the activity increased by GSSG was reduced to control levels by incubating with the corresponding sulfhydryl compounds such as GSH, cysteine and homocysteine respectively. Thus, microsomal glutathione S-transferase activity appears to be regulated by the formation and/or cleavage of a mixed disulfide bond between the sulfhydryl group present in the enzyme and GSSG. Therefore, the increase of microsomal glutathione S-transferase activity after phorone injection may be due to the formation of a mixed disulfide bond between the sulfhydryl group in the enzyme and GSSG.

Differential changes of glutathione S-transferase activity by dietary selenium

Dietary selenium deficiency produced increased activity of the glutathione S-transferases in the liver, kidney and duodenal mucosa. In these tissues, the residual activity of total glutathione peroxidase that included selenium-independent activity was considerably higher than that of selenium-dependent glutathione peroxidase. The enhanced activity of glutathione S-transferases was restored to control level 48 hr after an injection of selenite equivalent to the amount of daily selenium intake. Under the same conditions, selenium-dependent glutathione peroxidase activity increased with time and reached 11.9, 11.6 and 46.2% of the activity in the liver, kidney and duodenal mucosa of selenium-supplemented rats, respectively, 48 hr after selenite injection, whereas total glutathione peroxidase activity was not altered except in the kidney. These differential changes of glutathione S-transferase activity were intimately related to those of selenium-dependent glutathione peroxidase activity produced by selenium depletion and repletion, suggesting that the glutathione S-transferase activity was regulated by dietary selenium. Present findings support the idea that glutathione S-transferases having selenium-independent glutathione peroxidase activity function as a substitute for selenium-dependent glutathione peroxidase in selenium-deficient rats.

Catalytic action of selenium in the reduction of methemoglobin by glutathione

Abstract Selenite, selenate and selenocystine catalyzed the reduction of methemoglobin (metHb) by glutathione (GSH), while selenomethionine did not. Maximal reduction of metHb was observed with 10 −5 M selenite and 2 mM GSH, at pH 7.4. Selenite also catalyzed the reduction of metHb with cysteine or 2-mercaptoethylamine in place of GSH. Heavy metals and arsenite completely prevented the effect of selenite. These findings suggest that certain seleno-compounds catalyze the reduction of metHb by thiol compounds.

Acceleration of methemoglobin reduction in erythrocytes by selenium

Selenium accelerated the reduction of methemoglobin in erythrocytes. Its mode of action is suggested as a catalysis of the methemoglobin reduction by glutathione.

Stimulation of methemoglobin reduction by selenium: a comparative study with erythrocytes of various animals.

The extent of stimulation of methemoglobin (metHb) reduction by selenite depends upon the level of reduced glutathione (GSH) in the erythrocytes. The reason for the species difference in the effect of selenite was discussed with respect to species differences in the GSH levels in erythrocytes.

Anti-arthritic activity of bredinin, an immunosuppressive agent.

Bredinin has been found to have an inhibitory effect upon the secondary lesions occurring from adjuvant injection in rats.

Protective effect of selenite on nitrite toxicity.

Selenite was found to decrease nitrite-induced mortality in a dose-dependent manner. Its effect seems to be due to its action in reducing methemoglobin formed by nitrite.

Tissue-specific induction of intestinal glutathione S-transferases by αβ-unsaturated carbonyl compounds

Glutathione S-transferase activity in rat intestinal mucosa was increased by the injection of αβ-unsaturated carbonyl compounds such as phorone and diethylmaleate, but that in the liver and kidney was not. Since the administration of cycloheximide completely blocked the increase of the enzyme activity by phorone, the increase of the activity may be due to de novo synthesis rather than enzyme activation.