No abstract

Although the need for selenium in human and animal nutrition is well recognized, the question concerning the proper form of selenium for supplemental use is still being debated. Ideally, selenium should be supplemented in the form in which it occurs naturally in foods. Because the L-isomer of selenomethionine (Se-met) is a major natural food-form of selenium, synthetic L-Se-met or enriched food sources thereof such as selenium yeast are appropriate supplemental forms of Se for humans; for animals, DL-Se-met is acceptable. Ingested Se-met is either metabolized directly to reactive forms of selenium or stored in place of methionine in body proteins. Se-met metabolism is closely linked to protein turnover. At constant intakes in the nutritional range, tissue Se levels increase until a steady state is established, preventing the build-up to toxic levels.

Lithium is found in variable amounts in foods; primary food sources are grains and vegetables; in some areas, the drinking water also provides significant amounts of the element. Human dietary lithium intakes depend on location and the type of foods consumed and vary over a wide range. Traces of lithium were detected in human organs and fetal tissues already in the late 19th century, leading to early suggestions as to possible specific functions in the organism. However, it took another century until evidence for the essentiality of lithium became available. In studies conducted from the 1970s to the 1990s, rats and goats maintained on low-lithium rations were shown to exhibit higher mortalities as well as reproductive and behavioral abnormalities. In humans defined lithium deficiency diseases have not been characterized, but low lithium intakes from water supplies were associated with increased rates of suicides, homicides and the arrest rates for drug use and other crimes. Lithium appears to play an especially important role during the early fetal development as evidenced by the high lithium contents of the embryo during the early gestational period. The biochemical mechanisms of action of lithium appear to be multifactorial and are intercorrelated with the functions of several enzymes, hormones and vitamins, as well as with growth and transforming factors. The available experimental evidence now appears to be sufficient to accept lithium as essential; a provisional RDA for a 70 kg adult of 1,000 microg/day is suggested.

No abstract

Results of kinetic measurements are presented which indicate that the reactions of alkyl halides with vitamin B^,, cobaloximes(I), and other Co(I) chelates proceed by a classical Sn2 mechanism, the Co(I) centers being some of the most powerful nucleophiles known. The evidence for this mechanism providing the dominant reaction pathway is based mainly on the analysis of substrate structural effects on the substitution rates. Although the absolute reactivities of the Co(I) nucleophiles are up to 107 times greater than those, e.g., of iodide ion, the relative reactivities with various substrates are very similar. Surprisingly, the rates of reactions of the alkyl halides studied are no more sensitive to steric effects of the corrin ligand system than to those of the cobaloxime moiety. Steric hindrance by out-of-plane corrin ligands appear in later stages of the Co-C bond formation process, as evidenced by the instability of secondary alkyl cobalamins in contrast to the corresponding cobaloxime derivatives. The factors influencing the nucleophilicity of the Co(I) chelates, in particular, the effects of axial bases, ligand structure, and possible mechanistic alternatives of the alkylation reactions are discussed. The presence of the coordinated 5,6-dimethylbenzimidazole does not cause a substantial change of the Co(I) nucleophilicity of vitamin Bn"(5) G.