Specific Occurence of Selenium in Certain Enzymes and Amino Acid Transfer Ribonucleic Acids

Discoveries of vitamin B12 and selenium enzymes

My undergraduate education at Cornell University was followed by graduate studies on methane fermentations under the guidance of H.A. Barker at the University of California, Berkeley. My Ph.D. degree was granted in June 1949. Two anaerobic microorganisms isolated from the mud flats of San Francisco Bay served as sources of biochemical research material for later studies at the National Institutes of Health in Bethesda. These organisms, Methanococcus vannielii and Clostridium sticklandii, proved to be especially rich sources of selenium-dependent enzymes and seleno-tRNAs. New B12 coenzyme-dependent enzymes that catalyzed intermediate steps in the anaerobic conversion of lysine to fatty acids and ammonia were isolated from C. sticklandii and characterized. My research efforts since 1970 have dealt primarily with various aspects of selenium biochemistry. We have shown that selenium is an essential constituent of several enzymes in prokaryotes. Se is present in these either as a selenocysteine residue in the protein or alternatively, in a few molybdoenzymes, as a component of a bound cofactor. Recent studies with a human adenocarcinoma cell line led to the unexpected discovery that selenocysteine occurs in mammalian thioredoxin reductase. The selenium located in a redox center of this enzyme is essential for catalytic activity.

Bioavailability of selenium accumulated by selenite-reducing bacteria

The bioavailability of selenium (Se) was determined in bacterial strains that reduce selenite to red elemental Se (SeO). A laboratory strain of Bacillus subtilis and a bacterial rod isolated from soil in the vicinity of the Kesterson Reservoir, San Joaquin Valley, CA, (Microbacterium arborescens) were cultured in the presence of 1 mM sodium selenite (Na2SeO3). After harvest, the washed, lyophilized B. Subtilis and M. arborescens samples contained 2.62 and 4.23% total Se, respectively, which was shown to consist, within error, entirely of SeO. These preparations were fed to chicks as supplements to a low-Se, vitamin E-free diet. Three experiments showed that the Se in both bacteria had bioavailabilities of approx 2% that of selenite. A fourth experiment revealed that gray SeO had a bioavailability of 2% of selenite, but that the bioavailability of red SeO depended on the way it was prepared (by reduction of selenite). When glutathione was the reductant, bioavailability resembled that of gray SeO and bacterial Se; when ascorbate was the reductant, bioavailability was twice that level (3-4%). These findings suggest that aerobic bacteria such as B. subtilis and M. arborescens may be useful for the bioremediation of Se-contaminated sites, i.e., by converting selenite to a form of Se with very low bioavailability.

Embryotoxicity and dose-response relationships of selenium in hamsters

Pregnant hamsters were treated with selenite, selenate, and selenomethionine during the critical stages of embryogenesis. The dosing regimens were oral, intravenous, and osmotic minipump infusion. Malformations, mainly encephaloceles, were noted with oral and intravenous selenite and selenate but were associated with maternal toxicity manifested by inanition and weight loss. Fetal body weights and lengths were reduced in a dose-dependent manner with the inorganic forms. Single oral doses of selenomethionine above 77 mumol/kg induced similar malformations but not when the dose was delivered orally over four days nor by minipump over several days. Fetal body weights and lengths were decreased by selenomethionine in a dose-dependent manner. Maternal toxicity was pronounced with the higher doses of selenomethionine. Assigning a specific teratogenic effect to selenium is confounded by maternal toxicity.

Stop is not the end: physiological implications of translational readthrough

The translational readthrough mechanism permits the occasional misreading of termination codons by normal charged tRNAs causing extended translation beyond the stop signal. In both prokaryotes and eukaryotes translational readthrough is involved in the regulation of gene expression, as for example in the synthesis of the enzyme reverse transcriptase of the Murine Leukemia Virus (MuLV) (Yoshinaka et al., 1985). Here we particularly deal with the sensitivity of the translational readthrough process to two parameters which are affected by changes in physiological conditions: (1) fluctuations in the concentration of readthrough tRNAs and (b) The affinity of the tRNAs to termination codons. We also discuss the possible role of translational readthrough during major changes in cell physiology.

Isolation and characterization of a selenium metabolism mutant of Salmonella typhimurium

Selenium is a constituent in Escherichia coli of the anaerobic enzyme formate dehydrogenase in the form of selenocysteine. Selenium is also present in the tRNA of E. coli in the modified base 5-methylaminomethyl-2-selenouracil (mnm5Se2U). The pathways of bacterial selenium metabolism are largely uncharacterized, and it is unclear whether nonspecific reactions in the sulfur metabolic pathways may be involved. We demonstrated that sulfur metabolic pathway mutants retain a wild-type pattern of selenium incorporation, indicating that selenite (SeO32-) is metabolized entirely via selenium-specific pathways. To investigate the function of mnm5Se2U, we isolated a mutant which is unable to incorporate selenium into tRNA. This strain was obtained by isolating mutants lacking formate dehydrogenase activity and then screening for the inability to metabolize selenium. This phenotype is the result of a recessive mutation which appears to map in the general region of 21 min on the Salmonella typhimurium chromosome. A mutation in this gene, selA, thus has a pleiotropic effect of eliminating selenium incorporation into both protein and tRNA. The selA mutant appears to be blocked in a step of selenium metabolism after reduction, such as in the actual selenium insertion process. We showed that the absence of selenium incorporation into suppressor tRNA reduces the efficiency of suppression of nonsense codons in certain contexts and when wobble base pairing is required. Thus, one function of mnm5Se2U in tRNA may be in codon-anticodon interactions.