Dietary magnesium deficiency increases Gi alpha levels in the rat heart after myocardial infarction.

OBJECTIVE: Magnesium (Mg) is crucial for the function of G proteins which play important roles in mediating the inotropic effects of beta adrenergic agonists in the heart and are altered in heart failure. This study was performed to determine whether or not dietary Mg deficiency alters functional activity and levels of the two major ventricular G proteins, Gi alpha and Gs alpha in the heart after myocardial infarction (MI). METHODS: Six week old rats were fed an Mg adequate or deficient diet for 6 weeks. At the end of week 3, MI was induced by coronary artery ligation. A sham operation was performed as control. After surgery, surviving animals were maintained on their assigned diets for another 3 weeks. Then, cardiac function was measured, plasma and tissue were collected. RESULTS: Severe hypomagnesemia and increased plasma catecholamine level were observed in all animals fed the Mg deficient diet. A significant reduction of myocardial Mg concentration accompanied by elevated plasma and myocardial calcium concentrations was observed in MI animals with existing Mg deficiency vs. animals fed the Mg adequate diet. Cardiac function was impaired in MI rats and further reduced in MI rats with existing Mg deficiency. Gi alpha level was not altered by either Mg deficiency or MI alone, but was dramatically elevated in animals with combined Mg deficiency and MI (9.9 +/- 0.7 arbitrary unit.mg-1 protein) as compared to MI alone (5.8 +/- 0.6, P < 0.05) and Mg deficiency alone (6.1 +/- 0.8, P < 0.05). Gs alpha level did not differ between groups. GppNHp-, but not fluoride-stimulated adenylyl cyclase activity was slightly reduced in MI animals with existing Mg deficiency. CONCLUSION: The findings suggest that dietary Mg deficiency increases the expression of Gi alpha in the heart after MI, while levels and function of Gs alpha are not compromised during dietary Mg deficiency either with or without MI.

On the nature of selenium toxicity and carcinostatic activity.

Selenium toxicity was first confirmed in 1933 to occur in livestock that consumed plants of the genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya in the western regions of the United States. In 1957 selenium was identified as an essential nutrient for laboratory rats and soon thereafter for chickens and sheep. Essentiality for mammalian species was established in 1973 with the discovery that the enzyme glutathione peroxidase contained selenium. During this same period of time, human epidemiological evidence suggested that selenium possessed anticarcinogenic effects. Since the 1970s, many animal studies have confirmed the human epidemiologic evidence that selenium compounds possess carcinostatic activity. Less progress has been made in explaining why many of these compounds of selenium are toxic and why these same compounds are carcinostatic. In 1988 the observation was made that oxidation of glutathione by selenite produced superoxide, opening a new area for selenium research. This present paper, drawing information from the literature on selenium metabolism in plants and animals, selenium toxicology, selenium cytotoxicity, and selenium carcinostatic activity in animals over the last sixty years, sets forth a probable biochemical catalytic mechanism that encompasses both selenium toxicity and selenium carcinostatic activity. The thesis presented here for scrutiny is that compounds of selenium are toxic owing to their prooxidant catalytic activity to produce superoxide (O2.-), hydrogen peroxide, and very likely other cascading oxyradicals. The toxicity of selenium compounds is countered by plant and animal methylation reactions and antioxidant defenses. As carcinostasis is mostly known to occur at supranutritional levels of selenium in animals, carcinostasis appears to be directly correlated to selenium toxicity. The catalytic toxic selenium specie appears to be the metabolic selenide (RSe-) anion.

Cytolysis of human erythrocytes by a covalent antibody-selenium immunoconjugate.

A selenium (Se)-containing immunoconjugate of a human anti-erythrocyte membrane antibody (Ab-Se) has been synthesized via oxidation of the carbohydrate moieties of the antibody and covalent coupling with selenocystamine. The isolated Ab-Se immunoconjugate is shown to be more hemolytic than is selenocystamine when expressed on equivalent selenium basis. Native antibody preincubated with the human erythrocytes prevented hemolysis induced by the Ab-Se immunoconjugate. As observed microscopically, the Ab-Se immunoconjugate caused extensive damage to the erythrocyte membrane and lysis. The cytotoxicity of Se toward the human erythrocytes is believed to be caused initially by the localized generation of superoxide (O2.-) within the cell membrane. This is the first demonstration of site-directed immunoselectivity of Se cytotoxicity and demonstrates the potential for a free radical pharmacology based on localized Se-generated O2.-.

Selenium compounds have disparate abilities to impose oxidative stress and induce apoptosis.

The cancer chemopreventive effect of selenium cannot be fully accounted for by the role of selenium as a component of the antioxidant enzyme glutathione peroxidase, which suggests that chemoprevention occurs by another mechanism. Several studies have shown that thiol oxidation and free radical generation occur as a consequence of selenium catalysis and toxicity. In the present study, we evaluated three different selenium compounds; selenite, selenocystamine, and selenomethionine to determine the relative importance of the prooxidative effects of these compounds with regard to their ability to induce apoptosis. The experimental results suggest that, in addition to supporting an increased activity of glutathione peroxidase, an antioxidant function that the three selenium compounds did with equal efficacy, catalytic selenite, and selenocystamine generated 8-hydroxydeoxyguanosine DNA adducts, induced apoptosis and were found to be cytotoxic in mouse keratinocytes. The noncatalytic selenomethionine was not cytotoxic, did not generate 8-hydroxydeoxyguanosine adducts and did not induce cellular apoptosis at any of the selenium concentrations studied. In keratinocytes, apoptosis may be initiated by superoxide (O2*-) and oxidative free radicals that are generated by selenite and selenocystamine, but not by selenomethionine.

Inhibition of selenite-catalyzed superoxide generation and formation of elemental selenium (Se(o)) by copper, zinc, and aurintricarboxylic acid (ATA).

Selenite catalyzes the oxidation of glutathione (GSH) with the subsequent generation of superoxide (O2.-). Copper, zinc, and aurintricarboxylic acid (ATA) were tested for their ability to inhibit both the selenite-catalyzed generation of superoxide and the conversion of selenite to elemental selenium (Se(o)). As measured by lucigenin-dependent chemiluminescence (CL), copper, zinc, and ATA were shown to inhibit significantly (P < 0.05) selenite-catalyzed CL in a concentration-dependent manner. The inhibition of the selenium-catalyzed generation of superoxide by copper(II) was greater than by either zinc or ATA. In addition, Copper, zinc, and ATA all inhibited the conversion of selenite to Se(o). Inhibition of selenite-catalyzed CL by copper, zinc, and ATA is believed to occur as the result of inhibition of Se2- and/or GSSe-, the catalytic selenopersulfide anion of GSH.

Generation of reactive oxygen species from the reaction of selenium compounds with thiols and mammary tumor cells.

Sodium selenite, sodium selenate, selenocystine and selenomethionine were tested for their abilities to generate superoxide by the oxidation of glutathione and other thiols in the absence and presence of cells of the human mammary tumor cell line HTB123/DU4475. Free radical generation was measured by lucigenin- or luminol-amplified chemiluminescence. In the absence of tumor cells, lucigenin-dependent chemiluminescence was observed from the reaction of selenite with the thiols glutathione, 2-mercaptoethanol and L-cysteine, but not with oxidized glutathione. Superoxide dismutase, catalase, and glutathione peroxidase all suppressed the observed chemiluminescence; but when these enzymes were heat inactivated they had little suppressive inhibition on chemiluminescence. Luminol-dependent chemiluminescence from the reaction of selenite with glutathione was much less than that observed by lucigenin-amplified chemiluminescence. In the presence of the HTB123/DU4475 mammary tumor cells, lucigenin-dependent chemiluminescence was observed from the reactions of selenite and selenocystine with glutathione which were 5 and 23 times greater than their respective reactions with glutathione in the absence of tumor cells. The enhanced chemiluminescence generated by selenite and selenocystine in the presence of the tumor cells was also suppressed by superoxide dismutase, catalase and glutathione peroxidase. These data suggest that a free radical, the superoxide anion (O2-), and H2O2 are produced from the reaction of selenite and selenocystine with glutathione. These free radical reactions may account for the toxicity of selenite and selenocystine in vitro in comparison to a near absence of acute tumor cell toxicity and superoxide generation by selenate and selenomethionine with thiols. Enhanced chemiluminescence in the presence of tumor cells may be an expression of cellular selenium metabolism and the capability of cells to form selenium metabolites that more easily oxidize glutathione and other thiols producing reactive free radicals and peroxides.

In vitro hemolysis of rat erythrocytes by selenium compounds.

Rat erythrocytes were incubated in vitro with various selenium compounds at 37 degrees. Hemolysis occurred with some selenium compounds but not with corresponding sulfur analogues. Selenite induced more rapid loss of intracellular glutathione (GSH) than did selenocystine but was less hemolytic. Cystine caused neither loss of intracellular GSH nor hemolysis. Addition of GSH to the incubation medium enhanced hemolysis by selenite and selenium dioxide but inhibited hemolysis by selenocystine. Inclusion of glucose in the incubation medium also inhibited selenocystine-induced lysis of erythrocytes from both selenium-supplemented rats and selenium-deficient rats. The results suggest a relationship between the oxidation of intracellular GSH and the hemolysis by selenocystine, selenite and selenium dioxide.

Severe dietary magnesium deficiency does not alter levels and function of myocardial Gs alpha and Gi alpha.

Magnesium ions (Mg2+) play a crucial role in the activation and synthesis of guanine nucleotide-binding proteins (G proteins). However, there is no information about the influence of in vivo magnesium deficiency on the function and levels of G proteins. This study was done to investigate whether dietary magnesium deficiency alters function and levels of the two major myocardial G proteins, Gi alpha and Gs alpha. Severe hypomagnesemia and a significant reduction of myocardial magnesium occurred in rats fed a magnesium-deficient diet for 6 wk vs. rats fed a normal-magnesium diet (control). The magnesium-deficient rats developed focal myocardial lesions but their cardiac function was not impaired. Myocardial immunodetectable Gs alpha and Gi alpha levels of magnesium-deficient rats did not differ from control (Gs alpha: 2.39 +/- 0.52 vs. 2.76 +/- 0.72 arbitrary units/microgram protein, P > 0.05; Gi alpha: 1.60 +/- 0.52 vs. 1.89 +/- 0.30 arbitrary units/microgram protein, P > 0.05). Similarly, the function of Gs alpha and Gi alpha estimated by basal and ligand-stimulated adenylyl cyclase activity was not significantly different between the two groups of animals. The results show that dietary-derived magnesium deficiency sufficient to produce severe hypomagnesemia does not produce any significant change in levels or function of myocardial G proteins.

Hematologic data of selenium-deficient and selenium-supplemented rats.

Effect of dietary selenium as sodium selenite on in vivo hematological parameters of Sprague-Dawley rats was examined over a 7-month period. Dietary selenium did not alter total hemoglobin, hematocrits, erythrocyte counts, or the osmotic fragility pattern of rat blood. Selenium-excessive (1.0 ppm) rats showed slightly lower but not significantly lower methemoglobin levels than selenium-adequate (0.1 ppm) or selenium-deficient rats. Platelet counts tended to be higher in selenium-excessive rats and lower in selenium-deficient rats than in selenium-adequate rats, but the differences were not statistically significant. No clear trends were observed regarding the effect of dietary selenium on total leukocyte and differential leukocyte counts. After 7 months of dietary treatment blood glutathione peroxidase activity in selenium-deficient rats and in selenium-excessive rats was 16.8% and 142.2% of the activity in selenium-adequate rats. The results indicate that long-term selenium deficiency in rats produces no abnormal hematological parameters or any compensated hemolytic anemia in vivo.

Dietary selenium and aniline-induced methemoglobinemia in rats.

Depletion of selenium from rats for 8 weeks decreased blood glutathione peroxidase activity to 5.7% of that in selenium-supplemented (0.5 ppm selenium as Na2SeO3) rats. Aniline (60 mg/kg, i.p.) resulted in no significant difference in methemoglobin and blood reduced glutathione (GSH) levels between Se-deficient and Se-supplemented rats. A lowered aniline dose (36 mg/kg, i.p.) also resulted in no difference in methemoglobin levels. The selenium-deficient rat was able to reduce methemoglobin induced by aniline as efficiently as the selenium-sufficient rat.

Glutathione: is it an evolutionary vestige of the penicillins?

Glutathione is structurally similar to the precursor of the antibiotics found in the fungi of the genus Penicillium and Cephalosporium. The structural similarity is such to permit the rearrangement of glutathione to form the beta-lactam ring, common to many antibiotics. The possibility exists that glutathione may have evolved away from an antibiotic role in cells with the evolutionary development of specialized cells of immune systems.

Advances in the role of minerals in immunobiology.

Immunology and our understanding of its various cells, immunoglobulins and lymphokines are recent events that date from the work of Pasteur and Metchnikoff in the late nineteenth century. Experimental evidence has shown the importance of adequate dietary protein and vitamins. The present review examines past and recent experimental evidence for the role of minerals in the functioning of the immune system. Included is in vivo and in vitro information on the macrominerals; calcium and magnesium, the micro-(trace) minerals; iron, zinc, copper, and selenium as they affect various components of the immune system. The effects of gold as either gold-thiomaleate or gold-thioglucose on selenium is also reviewed.

The selenium content of selected meats, seafoods, and vegetables from Lubbock, Texas.

The selenium content of some frequently consumed foods from a commercial beef packer, a road-side vendor, and local stores in Lubbock, Texas was determined and compared to selenium data for the same or similar foods in the United States and from other countries. The comparative content of selenium in foods covered the period of time between 1970 and 1993. Our selenium analyses of foods show that on a fresh weight basis, the selenium content of seafoods > commercial beef > pork > ground beef > chicken. Cooking, air- or freeze-drying increased the selenium content of all foods significantly. When the selenium content of local foods is compared to similar foods in the United States and other countries, with few exceptions, there exists great uniformity in the selenium content in the major food groups, meats, fish, milk, and vegetables. New Zealand, known for its low soil selenium, had the lowest comparative food selenium content.

Effect of dietary copper on selenium toxicity in Fischer 344 rats.

The purpose of this study was to investigate the ameliorating effects of dietary copper supplementation on selenium toxicity. Nine groups (n = 6) of weanling Fischer 344 female rats were randomly assigned to treatment groups and fed diets containing nontoxic levels of copper as CuCl2 and/or selenium as selenite or selenocystamine. Weight gain, liver and spleen weights, plasma lipid peroxidation, and liver selenium and copper content were analyzed after the 6-wk treatment period. Concentrations of up to 10 times the daily lethal dose of dietary selenium were well tolerated in rats supplemented with dietary copper. As the dietary level of selenium was increased, the ratio of selenium to copper measured in the liver decreased. In the groups of rats in which dietary copper supplementation was absent and dietary selenium was supplemented, copper stores in the liver remained unchanged from control values. Copper's protective effects from dietary selenium toxicity may come from the formation of a copper-selenide complex that renders both selenium and copper metabolically unavailable and nontoxic.

Effect of selenium compounds and thiols on human mammary tumor cells.

The effect on cell viability and growth rate of sodium selenite, selenocystine, sodium selenate, and selenomethionine at selenium concentrations of 6.25 and 12.5 uM was studied in vitro on cells of the human mammary tumor cell line HTB123/DU4475. Selenite and selenocystine affected both cell viability and growth rate of the tumor cells at these selenium concentrations. Selenite and selenocystine decreased intracellular glutathione concentrations, but did not affect tumor cell glutathione peroxidase activity. After six days of exposure to either selenate or selenomethionine, the viability of tumor cells remained stable, but cell growth, as measured by numbers of cells, was retarded. Neither selenate nor selenomethionine produced changes in concentrations of intracellular glutathione. The toxic effect of selenite on tumor cells was enhanced by addition of 0.25 mM glutathione to the growth medium. Preincubation of the tumor cells with 62.5 uM buthionine sulfoximine decreased cellular glutathione to 15% of controls at 24 h and enhanced the toxicity of selenite toward the tumor cells. Glutathione, 2-mercaptoethanol, and L-cysteine were all toxic to the tumor cells in a dose-dependent manner.

Bioavailability of selenium from veal, chicken, beef, pork, lamb, flounder, tuna, selenomethionine, and sodium selenite assessed in selenium-deficient rats.

The bioavailability of selenium (Se) from veal, chicken, beef, pork, lamb, flounder, tuna, selenomethionine (SeMet), and sodium selenite was assessed in Se-deficient Fischer-344 rats. Se as veal, chicken, beef, pork, lamb, flounder, tuna, SeMet, and sodium selenite was added to torula yeast (TY) basal diets to comprise Se-inadequate (0.05 mg Se/kg) diets. Se as sodium selenite was added to a TY basal diet to comprise a Se-adequate (0.10 mg Se/kg), Se-control diet. The experimental diets were fed to weanling Fischer-344 rats that had been subjected to dietary Se depletion for 6 wk. After 9 wk of the dietary Se repletion, relative activity of liver glutathione peroxidase (GSHPx) from the different dietary groups compared with control rats (100%) was: flounder 106%, tuna 101%, pork 86%, sodium selenite 81%, SeMet 80%, beef 80%, chicken 77%, veal 77%, and lamb 58%. Se from flounder was the most efficient at restoring Se concentrations in the liver and skeletal muscle. Se from sodium selenite, SeMet, beef, veal, chicken, pork, lamb, and tuna was not dietarily sufficient to restore liver and muscle Se after 9 wk of recovery following a 6-wk period of Se depletion.

Free radical generation by selenium compounds and their prooxidant toxicity.

Selenium (Se) and many of its compounds are among the most toxic of nutrients. Selenium toxicity was first described in range animals in the western United States in the 1930's which consumed "selenium accumulator" plants of the genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya. Selenites and selenates from the soil accumulate in these plants primarily as methylated selenium compounds and plants evolve dimethyldiselenide and dimethylselenide. Dietary selenium, primarily as selenomethionine and selenocysteine for humans fulfill the dietary requirement for selenoenzymes and proteins. In humans and animals excessive dietary selenium may be toxic. In vitro, selenium compounds such as selenite, selenium dioxide and diselenides react with thiols, such as glutathione, producing superoxide and other reactive oxygen species. This catalytic reaction of selenium compounds with thiols likely accounts for selenium toxicity to cells ex vivo and in vivo where the major glutathione producing organ, the liver, is also the major target organ of selenium toxicity. Selenium enzymes and selenoethers that do not readily form a selenide (RSe-) anion and compounds such as Ebselen where selenium is sequestered, are not toxic. Methylation of selenium by both plants and animals serves to detoxify selenium by generating methylselenides. Alternatively, full reduction of Se to elemental selenium (Se0) as done by some bacteria and the formation of heavy metal selenides such as Ag2Se or Hg2Se, results in a non-catalytic non-toxic form of selenium. This catalytic prooxidant attribute of some selenium compounds appears to account for its toxicity when such activity exceeds plant and animal methylation reactions and antioxidant defenses. This prooxidant activity may also account for cellular apoptosis and may provide a useful pharmaceutical application for selenium compounds as antibacterial, antiviral, antifungal and anticancer agents.

In vitro evidence for a relationship between magnesium and vitamin B-6.

Vitamin B-6 and magnesium appear to interact in vivo and it has been suggested that vitamin B-6 may enhance transport or accumulation of magnesium in cells. The purpose of this study was to evaluate the possibility of the formation of B-6 vitamer-Mg complexes in vitro using spectrophotometric analysis of ultraviolet or visible spectra or change in fluorescence intensity of B-6 vitamers. Pyridoxal phosphate but not pyridoxal appears to form a complex with Mg as evidenced by a change in the ultraviolet and visible spectra with an increase in absorption and spectral shift in the visible maxima from 389 to 388 nm and a decrease in the ultraviolet absorption maxima at 296 nm. Addition of Mg to pyridoxal phosphate but not the other B-6 vitamers enhanced the fluorescent intensity of pyridoxal phosphate at least threefold and evoked a spectral shift in both the fluorescence excitation and emission maximum. Electrochemical titration of pyridoxal phosphate in the presence of Mg affects the pKa- of the phenolic hydroxyl and the secondary phosphate ionization. These data collectively indicate that pyridoxal phosphate, but not pyridoxal, appears to form a coordinated complex with Mg and the coordination site is likely to be related to the primary phosphate and aldehyde moiety of pyridoxal phosphate.