Raymond F. Burk

Bio: Raymond F. Burk is an academic researcher from Vanderbilt University. The author has contributed to research in topics: Selenoprotein P & Selenium deficiency. The author has an hindex of 73, co-authored 181 publications receiving 19530 citations. Previous affiliations of Raymond F. Burk include University of Texas Southwestern Medical Center & United States Department of Veterans Affairs.

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

Abstract: 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.

SELENOPROTEIN P: An Extracellular Protein with Unique Physical Characteristics and a Role in Selenium Homeostasis

Abstract: Selenoprotein P is an abundant extracellular glycoprotein that is rich in selenocysteine. It has two domains with respect to selenium content. The N-terminal domain of the rat protein contains one selenocysteine residue in a UxxC redox motif. This domain also has a pH-sensitive heparin-binding site and two histidine-rich amino acid stretches. The smaller C-terminal domain contains nine selenocysteine and ten cysteine residues. Four isoforms of selenoprotein P are present in rat plasma. They share the same N terminus and amino acid sequence. One isoform is full length and the three others terminate at the positions of the second, third, and seventh selenocysteine residues. Selenoprotein P turns over rapidly in rat plasma with the consequence that approximately 25% of the amount of whole-body selenium passes through it each day. Evidence supports functions of the protein in selenium homeostasis and oxidant defense. Selenoprotein P knockout mice have very low selenium concentrations in the brain, the testis, and the fetus, with severe pathophysiological consequences in each tissue. In addition, those mice waste moderate amounts of selenium in the urine. Selenoprotein P binds to endothelial cells in the rat, and plasma levels of the protein correlate with prevention of diquat-induced lipid peroxidation and hepatic endothelial cell injury. The mechanisms of these apparent functions remain speculative and much work on the mechanism of selenoprotein P function lies ahead. Measurement of selenoprotein P in human plasma has shown that it is depressed by selenium deficiency and by cirrhosis. Selenium supplementation of selenium-deficient human subjects showed that glutathione peroxidase activity was optimized before selenoprotein P concentration was optimized, indicating that plasma selenoprotein P is the better index of human selenium nutritional status.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Abstract: Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

[44] Glutathione peroxidase

Abstract: Publisher Summary This chapter presents a procedure for the preparation of glutathione peroxidase, which is regarded as a major protective system against endogenously and exogenously induced lipid peroxidation. Two types of methods are used for determining the activity of glutathione peroxidase. One involves a direct measurement of unconsumed glutathione (GSH) at fixed time periods by polarographic GSH analysis' (Method 1), or by the dithionitrobenzoic acid method (Method 2). The second approach takes advantage of the capability of glutathione reductase, with nicotinamide adenine dinucleotide phosphate (NADPH), to regenerate GSH from oxidized GSH. The decrease in NADPH is continuously measured spectrophotometrically, while the GSH concentration in the enzymatic cycle remains essentially constant (Method 3). A convenient source for the preparation of glutathione peroxidase is bovine blood including the following steps: hemolysate; organic solvent precipitation; phosphate precipitation; absorption to phenyl-sepharose; and washing on diethylaminoethyl (DEAE)–sephadex, S-300 sephacryl, and hydroxylapatite column.

Effects of Selenium Supplementation for Cancer Prevention in Patients With Carcinoma of the Skin: A Randomized Controlled Trial

Abstract: Objective. —To determine whether a nutritional supplement of selenium will decrease the incidence of cancer. Design. —A multicenter, double-blind, randomized, placebo-controlled cancer prevention trial. Setting. —Seven dermatology clinics in the eastern United States. Patients. —A total of 1312 patients (mean age, 63 years; range, 18-80 years) with a history of basal cell or squamous cell carcinomas of the skin were randomized from 1983 through 1991. Patients were treated for a mean (SD) of 4.5 (2.8) years and had a total follow-up of 6.4 (2.0) years. Interventions. —Oral administration of 200 μg of selenium per day or placebo. Main Outcome Measures. —The primary end points for the trial were the incidences of basal and squamous cell carcinomas of the skin. The secondary end points, established in 1990, were all-cause mortality and total cancer mortality, total cancer incidence, and the incidences of lung, prostate, and colorectal cancers. Results. —After a total follow-up of 8271 person-years, selenium treatment did not significantly affect the incidence of basal cell or squamous cell skin cancer. There were 377 new cases of basal cell skin cancer among patients in the selenium group and 350 cases among the control group (relative risk [RR], 1.10; 95% confidence interval [CI], 0.95-1.28), and 218 new squamous cell skin cancers in the selenium group and 190 cases among the controls (RR, 1.14; 95% CI, 0.93-1.39). Analysis of secondary end points revealed that, compared with controls, patients treated with selenium had a nonsignificant reduction in all-cause mortality (108 deaths in the selenium group and 129 deaths in the control group [RR, 0.83; 95% CI, 0.63-1.08]) and significant reductions in total cancer mortality (29 deaths in the selenium treatment group and 57 deaths in controls [RR, 0.50; 95% CI, 0.31-0.80]), total cancer incidence (77 cancers in the selenium group and 119 in controls [RR, 0.63; 95% CI, 0.47-0.85]), and incidences of lung, colorectal, and prostate cancers. Primarily because of the apparent reductions in total cancer mortality and total cancer incidence in the selenium group, the blinded phase of the trial was stopped early. No cases of selenium toxicity occurred. Conclusions. —Selenium treatment did not protect against development of basal or squamous cell carcinomas of the skin. However, results from secondary end-point analyses support the hypothesis that supplemental selenium may reduce the incidence of, and mortality from, carcinomas of several sites. These effects of selenium require confirmation in an independent trial of appropriate design before new public health recommendations regarding selenium supplementation can be made.