Molecular biology of selenium with implications for its metabolism.

Selenium has a highly specific metabolism centered around its incorporation as selenocysteine into selenoproteins. An outline of this metabolism has emerged from recent molecular biological and biochemical studies of bacteria and animals. A unique tRNA, designated tRNA[Ser]Sec, is charged with L-serine, which is then converted through at least two steps to selenocysteine. With the aid of a unique translation factor, the selenocysteinyl-tRNA[Ser]Sec recognizes specific UGA codons in mRNA to insert selenocysteine into the primary structure of selenoproteins. Turnover of selenoproteins presumably liberates selenocysteine which is toxic in its free form. Selenocysteine beta-lyase catabolizes free selenocysteine and makes its selenium available for reuse. Proteins contain almost all the selenium in animals. Of the known selenoproteins, the glutathione peroxidases contain the most selenium. Cellular and plasma glutathione peroxidases are products of different genes but have 44% identity of amino acid sequence. There is evidence for other proteins of this family. Selenoprotein P is an unrelated protein with multiple selenocysteines in its primary structure. It contains most of the selenium in rat plasma. Studies of the regulation of cellular glutathione peroxidase by selenium have yielded conflicting results, but there is a strong suggestion that mRNA levels of the rodent liver glutathione peroxidase decrease in selenium deficiency. This could be a mechanism for directing selenium to the synthesis of other selenoproteins. Although present knowledge allows construction of an outline of selenium metabolism, several steps have not been characterized and little is known about mechanisms of its regulation.     

Substructure of the glomerular slit diaphragm in freeze-fractured normal rat kidney

In the renal glomerulus, the narrow slits between adjacent epithelial podocytes are bridged by a diaphragm (2, 8, 11). In rat and mouse kidneys fixed by perfusion with tannic acid and glutaraldehyde (TAG), it has recently been discovered that this diaphragm has a highly ordered, isoporous substructure (9). It consists of a regular array of alternating cross bridges extending from the podocyte plasma membranes to a centrally running filament. This zipperlike pattern results in two rows of rectangular pores, approximately 40 X 140 A in cross section, dimensions consistent with the proposed role of the diaphragm as an important filtration barrier to plasma proteins (6). In the present study, we found in freeze-cleaved and in freeze-etched normal rat glomeruli that the surface of the slit diaphragm has an appearance conforming to the pattern found in sectioned material.     

Glutathione peroxidase activity in selenium-deficient rat liver

Glutathione peroxidase activity in the liver supernatant from rats fed a Se-deficient diet for 2 weeks was 8% of control when measured with H₂O₂ but 42% of control when assayed with cumene hydroperoxide. Two peaks of glutathione peroxidase activity were present in the Sephadex G-150 gel filtration chromatogram of rat liver supernatant when 1.5 mM cumene hydroperoxide was used as substrate. Only the first peak was detected when 0.25 mM H₂O₂ was used as substrate. The first peak was absent from chromatograms of Se-deficient rat liver supernatants; but the second peak, which eluted at a position corresponding to M.W. = 39,000, appeared unchanged. The second peak thus represents a second glutathione peroxidase activity which catalyzes the destruction of organic hydroperoxides but has little activity toward H₂O₂ and which persists in severe selenium deficiency.     

More than skin deep: a self-consistency approach to the psychology of cosmetic surgery

Underlying attitudes about the general self and the specific body part operated on in cosmetic surgery were investigated. It was hypothesized that female cosmetic surgery patients would feel less favorably toward their noses, faces, or breasts than toward their overall self. These marked inconsistencies would cause "normal" individuals to seek practical solutions of enhancing the esteem of the particular body part, to make it consistent with their general view of themselves. Forty female cosmetic surgery patients were tested before and 2 and 4 months after surgery. In all, 12 hypotheses were made within the general self-consistency framework and 11 were upheld at levels ranging from 0.02 to 0.001. Self-consistency theory accurately represents the female cosmetic surgery patient as a normal woman in terms of self-esteem who is attempting to remediate a consciously felt inconsistency between general and specific body-part esteem. Cosmetic surgery seems to reduce this inconsistency.     

Selenium deficiency, endurance exercise capacity, and antioxidant status in rats

Increased O2 metabolism imposed by physical exercise is likely to augment the production of active O2 species that have been shown to react with lipids, proteins, and DNA. Antioxidants and antioxidant enzymes, such as the selenium enzyme glutathione peroxidase, minimize or prevent such potentially toxic reactions. This study shows that selenium deficiency decreases glutathione peroxidase activity in liver and muscle (less than 80%, P less than 0.001), increases total glutathione in liver, muscle, and plasma (P less than 0.05) and increases muscle cytochrome oxidase activity, and ubiquinone content (P less than 0.05) but has no effect on endurance capacity. Exercise to exhaustion resulted in a significant (P less than 0.001) elevation of total and oxidized glutathione (GSSG) and a significant (P less than 0.05) decrease of vitamin E in plasma of control and selenium-deficient rats. Acute exercise also increased tissue GSSG levels in both control and selenium-deficient groups of rats. Hence, despite a large depletion of selenium-deficient glutathione peroxidase, pronounced oxidation of glutathione to GSSG can be produced by the increased oxidative metabolism during physical exercise. The results suggest that the residual glutathione peroxidase activity is sufficient to detoxify hydroperoxides in exercising selenium-deficient animals and to prevent the impairment of endurance capacity.     

Cytoplasmic male sterility in Nicotiana, restoration of fertility, and the nucleolus

Plants with the cytoplasm of Nicotiana repanda and the chromosomes of N. tabacum produce nonfunctional and feminized anthers. Introduction of a satellited fragment chromosome, apparently derived from N. repanda, restores normal anthers and pollen fertility. The fragment is somatically stable and addition-homozygotes transmit it to the great majority of their offspring. Cells with the fragment exhibit amphiplasty, i.e. , in plants having one or two fragments, nucleolar organizers of N. tabacum are suppressed and the nucleoli are entirely or largely produced by the fragments. Formation of nucleoli by organizers from N. repanda in N. repanda cytoplasm may thus be a condition for male fertility. The manner in which nucleoli may influence the development of originally indeterminate sex primordia is discussed. Further cytological studies of cytoplasmically male-sterile plants and restorer chromosomes are needed to decide whether the observed relationship is of functional significance or merely accidental.     

A comparison of methods for the measurement of protein turnover in vivo.

Steady-state rates of turnover of two single proteins were measured in vivo by two independent methods. The fractional rate of synthesis of liver ornithine aminotransferase, measured by a continuous infusion of L-[2,6-3H]tyrosine, was 0.42 day-1, whereas in the same animals the fractional rate of degradation measured by loss of radioactivity from amino acids labelled via [14C]bicarbonate was 0.40 day-1. The agreement between methods confirms the reliability of each method for the study of hepatic protein turnover. In contrast, [14C]bicarbonate-labelled amino acids are extensively reutilized in muscle, and are therefore unsuitable for measuring rates of muscle protein breakdown.     

Studies on the mechanism for entry of vesicular stomatitis virus glycoprotein g mRNA into membrane-bound polyribosome complexes

Glycoprotein mRNA (G mRNA) of vesicular stomatitis virus is synthesized in the cytosol fraction of infected HeLa cells. Shortly after synthesis, this mRNA associates with 40S ribosomal subunits and subsequently forms 80S monosomes in the cytosol fraction. The bulk of labeled G mRNA is then found in polysomes associated with the membrane, without first appearing in the subunit or monomer pool of the membrane-bound fraction. Inhibition of the initiation of protein synthesis by pactamycin or muconomycin A blocks entry of newly synthesized G m RNA into membrane-bound polysomes. Under these circumstances, labeled G mRNA accumulates into the cytosol. Inhibition of the elongation of protein synthesis by cucloheximide, however, allows entry of 60 percent of newly synthesized G mRNA into membrane-bound polysomes. Furthermore, prelabeled G mRNA associated with membrane-bound polysomes is released from the membrane fraction in vivo by pactamycin or mucomycon A and in vitro by 1mM puromycin - 0.5 M KCI. This release is not due to nonspecific effects of the drugs. These results demonstrate that association of G mRNA with membrane-bound polysomes is dependent upon polysome formation and initiation of protein synthesis. Therefore, direct association of the 3' end of G mRNA with the membrane does not appear to be the initial event in the formation of membrane-bound polysomes.