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Biomedical subjects

R F Burk

Publications and source records attributed to R F Burk.

At least 73 records · Page 4Linked to original sources
PubMed1990

Selenium and amino acid composition of selenoprotein P, the major selenoprotein in rat serum.

Selenoprotein P is the second plasma selenoprotein to be purified. It is a glycoprotein and has been shown to be distinct from plasma glutathione peroxidase. This study characterizes selenoprotein P further. Deglycosylation of the protein shifts its migration on sodium dodecyl sulfate-polyacrylamide gel electrophoresis from Mr 57,000 to Mr 43,000, indicating it has a substantial carbohydrate component. Measurement of selenium indicates a selenium content of 7.5 +/- 1.0 atoms/molecule based on a polypeptide weight of 43,000. Amino acid analysis accounts for all the selenium as selenocysteine. The protein is also rich in cysteine (17 residues) and histidine (23 residues). Fragmentation of selenoprotein P by trypsin and by cyanogen bromide produces peptides with varying selenium content. This indicates that selenium-rich regions of the protein exist. The concentration of selenoprotein P determined by radioimmunoassay in serum from control rats is 26.3 +/- 4.5 micrograms/ml and in serum from selenium-deficient rats it is 2.7 +/- 0.8 micrograms/ml. Depletion of selenoprotein P from control serum using an immunoaffinity column indicates that over 60% of serum selenium in the rat is contained in this protein. These results demonstrate that selenoprotein P is the major form of selenium in rat serum. It is the first selenoprotein described which has more than one selenium atom/polypeptide chain.

Amino Acids↗
PubMed1990

Effect of selenium deficiency on tissue taurine concentration and urinary taurine excretion in the rat.

The purpose of this study was to determine the effect of selenium deficiency on tissue taurine levels and urinary taurine excretion. Weanling male Sprague-Dawley rats were fed selenium-deficient or selenium-adequate diets for 20 weeks. As selenium deficiency developed, urinary taurine excretion increased in selenium-deficient rats compared to controls. At 12 weeks, the selenium-deficient rats excreted 1.7-fold more taurine than control rats. At the same time plasma glutathione peroxidase was 1.2% of control and plasma glutathione was 226% of control. At 20 weeks, renal taurine was decreased but renal glutathione was increased in selenium-deficient rats compared to controls. Feeding the experimental diet for 6 weeks without methionine supplementation caused a fall in urinary taurine excretion. However, there was no difference between selenium-deficient and control rats. These results indicate that selenium deficiency affects renal handling of taurine in the rat when dietary sulfur amino acids are not restricted.

Journal Article↗
PubMed1990

A series of prostaglandin F2-like compounds are produced in vivo in humans by a non-cyclooxygenase, free radical-catalyzed mechanism.

Increasing attention has focused on the role of free radicals derived from oxygen in the pathophysiology of a wide variety of disorders. One of the well-recognized targets of free radical-induced injury is peroxidation of lipids. Using a variety of approaches, we have found that a series of prostaglandin F2-like compounds are produced in vivo in humans by a non-cyclooxygenase mechanism involving free radical-catalyzed peroxidation of arachidonic acid. Levels of these compounds in normal human plasma and urine range from 5 to 40 pg/ml and 500 to 4000 pg/mg of creatinine, respectively. In rats, their formation was found to increase as much as 200-fold in association with marked free radical-catalyzed lipid peroxidation induced by administration of CCl4 and diquat. To explore whether these prostanoids can exert biological activity, the effects of one of the compounds formed by this mechanism, 8-epi-prostaglandin F2 alpha, was examined in the kidney in the rat. Infusion of 8-epi-prostaglandin F2 alpha into a peripheral vein (5 micrograms/kg per min) or intrarenally (0.5-2.0 micrograms/kg per min) resulted in marked parallel reductions in renal blood flow and glomerular filtration rate. That the formation of these prostanoids is catalyzed by free radicals and that they can exert potent biological activity suggest that these prostanoids may participate as pathophysiological mediators in oxidant injury. Quantification of these compounds may also provide a noninvasive approach to assess oxidant status in humans. That the formation of these prostanoids occurs independent of the catalytic activity of the cyclooxygenase enzyme suggests that there may be limitations at times regarding the reliability of the use of cyclooxygenase inhibitors to assess the role of prostaglandins in certain pathophysiological processes.

Animals↗
PubMed1989

Nitroglycerin and isosorbide dinitrate stimulation of glutathione disulfide efflux from perfused rat liver.

Nitroglycerin (GTN) and isosorbide dinitrate (ISD) are metabolized by glutathione S-transferase to nitrite with production of GSSG from GSH. Infusion of organic nitrates into perfused rat liver led to efflux of GSSG in the bile and nitrite in the perfusate. Biliary GSSG increased more rapidly than did nitrite release as GTN infusion rate was increased, indicating that GSSG reducing capacity was being exceeded. Rapid GTN-induced oxidation of GSH may be the mechanism of tissue GSH depletion by GTN and other alkylnitrates. Such depletion of glutathione may reduce nitrite production from organic nitrates and underlie tolerance to these drugs.

Animals↗
PubMed1989

Diastolic dysfunction of perfused rat hearts induced by hydrogen peroxide. Protective effect of selenium.

Oxidant substances such as hydrogen peroxide are postulated to cause cardiac dysfunction and injury in a number of pathological conditions. Selenium is an essential nutrient which serves as an oxidant defense through the selenoenzyme glutathione peroxidase. This enzyme metabolizes hydrogen peroxide; its activity in rat heart is reduced to 5% of control by selenium deficiency. Left ventricular function of selenium-deficient and control rat hearts was studied in a Langendorff preparation under isovolumic conditions. A stabilization period of 20 min was followed by a 70 min infusion of hydrogen peroxide at 375 or 1500 nmol/min. When no hydrogen peroxide was infused, perfusion for 90 min had no effect on systolic or diastolic function and no effect of selenium deficiency was detected. Hydrogen peroxide infusion into selenium-deficient hearts at 375 nmol/min led to impaired isovolumic relaxation and a substantial increase in end-diastolic pressure after 45 min which worsened progressively until the experiment was terminated. By contrast no effect was observed on systolic contractile function as assessed by peak pressure or developed pressure. Infusion of this dose of hydrogen peroxide into control hearts had no significant effect on diastolic or systolic function. However, infusion of 1500 nmol hydrogen peroxide/min into control hearts caused diastolic dysfunction after 30 min without affecting systolic function. These results indicate that hydrogen peroxide injury to the perfused rat heart is manifested by diastolic dysfunction before systolic dysfunction occurs. Selenium deficiency lowers the dose of hydrogen peroxide needed to cause diastolic dysfunction. This suggests that the selenoenzyme glutathione peroxidase protects the heart against hydrogen peroxide injury.

Animals↗
PubMed1989

Dietary selenium intake controls rat plasma selenoprotein P concentration.

The purpose of this study was to determine the effect of dietary selenium on selenoprotein P concentration. Selenoprotein P was quantitated in plasma by radioimmunoassay. Selenium-dependent glutathione peroxidase activity in plasma and liver 105,000 x g supernatant was measured for comparison. Weanling male rats were fed a selenium-deficient diet or a control diet that contained 0.5 mg selenium/kg as Na2SeO4. The concentration of selenoprotein P fell at approximately the same rate in the rats fed the selenium-deficient diet as did plasma glutathione peroxidase activity. Groups of weanling rats were fed different levels of selenium for 8 wk. Selenoprotein P concentration was proportional to dietary selenium level up to 0.1 mg/kg and was a greater percentage of control values than was glutathione peroxidase activity. No increment in selenoprotein P concentration occurred between 0.1 and 0.5 mg selenium/kg diet. These results indicate that the concentration of selenoprotein P in the plasma is directly dependent on selenium supply in the diet up to 0.1 mg/kg. There is overlap between the dietary selenium ranges in which selenoprotein P concentration and glutathione peroxidase activity increase, but the selenoprotein P range is lower than the glutathione peroxidase range.

Administration, Oral↗
PubMed1989

Recent developments in trace element metabolism and function: newer roles of selenium in nutrition.

Until recently, studies of the function of selenium focused on the selenoenzyme, glutathione peroxidase. However, the recognition that several metabolic effects of selenium are not associated with glutathione peroxidase has forced a re-evaluation of the function of this enzyme and the element. Hepatic glutathione peroxidase contains a significant percentage of the regulated selenium in the rat and is more sensitive to selenium deficiency than other selenoproteins. Thus, in addition to its enzymatic activity, it might have a storage function for the element. Another selenoprotein, designated selenoprotein P, has been found in rat plasma and has been quantitated. Its function is not yet known, but it has been postulated to be a transport protein for selenium and a defense against oxidant stress. Understanding the nutritional effects of selenium will require better characterization of glutathione peroxidase, selenoprotein P and other selenoproteins.

Animals↗
PubMed1989

Biochemical studies of a selenium-deficient population in China: measurement of selenium, glutathione peroxidase and other oxidant defense indices in blood.

Selenium deficiency is necessary for the development of the cardiomyopathy known as Keshan disease. Healthy boys and men (19-22 per group) from a low selenium area (Dechang County) and from an area where sodium selenite was added to salt (Mianning County) were studied. Keshan disease was endemic in Dechang but occurred rarely in Mianning. After an initial blood sampling, each subject received daily selenium supplements (100 micrograms selenium for boys and 200 micrograms for men) as sodium selenate for 14 d. Blood was sampled again at 7 and 14 d. Boys from Dechang had blood selenium levels similar to levels reported for patients with Keshan disease. Plasma glutathione peroxidase activity in boys and men from Dechang was 33 and 43%, respectively, of values from the corresponding groups in Mianning. Comparison of plasma selenium concentrations in boys and men from Dechang gave values of 33 and 38%, respectively, of the corresponding groups in Mianning. Selenium status did not affect red blood cell superoxide dismutase or catalase activities. Plasma vitamin E concentration was below the normal range in all groups but was unaffected by selenium status. Measurements of plasma malondialdehyde revealed no difference between subjects from Dechang and subjects from Mianning. Selenium supplementation raised plasma glutathione peroxidase activity and plasma selenium concentration in all groups. Groups with higher plasma selenium concentration had relatively smaller increases in glutathione peroxidase activity than in selenium concentration. These results characterize the selenium deficiency in subjects at risk for developing Keshan disease. The results obtained with supplementation of selenium indicate the presence of additional plasma forms of selenium besides glutathione peroxidase.

Adolescent↗
PubMed1989

Tissue and biliary glutathione disulfide in the perfused vitamin E-deficient rat liver.

Vitamin E and glutathione protect against oxidative damage in vivo. In this study the relationship between these two defenses has been examined in the isolated perfused rat liver. The activities of glutathione reductase and glutathione S-transferase were unaffected by vitamin E deficiency, while glutathione peroxidase activity was decreased slightly. The glutathione redox status of vitamin E-deficient and control livers was assessed. GSSG was slightly higher in vitamin E-deficient livers (70 +/- 5 nmol GSH equivalents/g liver) than in controls (56 +/- 3 nmol GSH equivalents/g liver) under basal conditions. However, biliary GSSG release was 41% lower in vitamin E-deficient livers (0.46 +/- 0.08 nmol GSH equivalents/g liver.min) than in controls (0.78 +/- 0.23 nmol GSH equivalents/g liver.min). Inhibition of GSSG reduction by BCNU raised liver and biliary GSSG by a similar amount in vitamin E-deficient and control livers. Thus biliary GSSG efflux, a frequently used index of oxidant stress, is not increased in vitamin E-deficient perfused livers compared with control. Therefore, in the perfused rat liver model, no evidence was obtained that vitamin E deficiency activates the hepatic glutathione system.

Animals↗
PubMed1988

Inhibition of CCl4 metabolism by oxygen varies between isoenzymes of cytochrome P-450.

Oxygen inhibition of CCl4 metabolism by different isoenzymes of cytochrome P-450 was assessed by studying liver microsomes isolated from control rats and rats treated with phenobarbital or isoniazid. Rates of CCl4 metabolism were similar for all microsomes under a nitrogen atmosphere. An air atmosphere inhibited metabolism by microsomes from control rats to 12% of the value under nitrogen and metabolism by microsomes from rats treated with phenobarbital to 5%. It inhibited metabolism by microsomes from rats treated with isoniazid only to 32%. Rats treated with phenobarbital, which increases hepatic cytochrome P-450 content, or isoniazid, which does not increase hepatic cytochrome P-450 content, both metabolized more CCl4 than control rats as indicated by exhalation of greater quantities of CCl4 metabolites and by an increase in CCl4 toxicity. These results indicate that some isoenzymes of cytochrome P-450 are more effective than others in metabolizing CCl4 when oxygen is present.

Animals↗
PubMed1988

Formation of glutathione adducts of carbon tetrachloride metabolites in a rat liver microsomal incubation system.

Metabolism of CCl4 by rat liver microsomes causes damage to the membrane. GSH diminishes that damage. One mechanism of GSH protection has been characterized. It involves formation of diglutathione carbonate from two molecules of GSH and one molecule of phosgene, an oxygenated metabolite of CCl4. The present studies were done to seek other GSH adducts of CCl4 metabolites and to examine the effect of oxygen tension on their formation. Incubations were carried out in sealed flasks under oxygen concentrations ranging from 0.14 to 21% at 37 degrees. The CCl4 concentration was 72 microM. 14CCl4 and 35S-GSH were used to label metabolites. High pressure liquid chromatographic analysis of the aqueous phase demonstrated two GSH adducts of CCl4 metabolites. One adduct was oxygen-dependent and was identified as diglutathione carbonate by its co-elution with a diglutathione carbonate standard. Its formation showed no evidence of saturation when GSH concentrations as high as 10 mM were used, indicating that the overall process was nonenzymatic. Formation of the other adduct was greatest under the lowest oxygen concentration studied and none occurred at oxygen tensions of 5% or greater. Based on experiments with radiolabeled CCl4 and GSH, this metabolite appeared to be a product of one molecule each of CCl4 and GSH. Formation of this adduct had enzymatic characteristics. It was saturable with respect to GSH with an apparent Km of 70 microM, and other thiol compounds that were tested could not substitute for GSH. The adduct was unstable during isolation attempts and was not characterized further. Formation of these two GSH adducts could account for some of the protection by GSH against CCl4 injury.

Animals↗