The role of arsenic in methyl metabolism and genetic expression; identification of a reliable indicator of arsenic status in humans.
Because relatively low serum arsenic concentrations have been associated with vascular diseases and central nervous system injury, more systematic investigation of the possible role of arsenic in these disorders.
Of the five minerals discussed in this chapter, boron has received the most extensive study of its possible nutritional importance for animals and humans. Still, the collective body of evidence has yet to establish a clear biological function for boron in humans. There is evidence that boron is required by vascular plants and some microorganisms. The only known boron-containing compounds in nature are organoboron complexes from plants, some of which may have antibiotic properties (Hunt, 1998; Nielsen, 1997). Principles of bioinorganic chemistry predict that boron, which is primarily in the form of boric acid, B(OH)3, at physiological pH, binds to cis-diols, perhaps with some specifically, and forms condensation products that are moderately labile in aqueous solutions (da Silva and Williams, 1991). The latter could theoretically provide stability to diol-rich molecules such as polysaccharides or steroids. Boron can act as an inhibitor of activity for a wide variety of enzymes in vitro (Hunt, 1998). However, no boron-containing enzyme has been identified.
In higher animals, boron has not been shown to have a sufficiently definitive pattern of effects to establish a function. Embryonic defects related to boron depletion have been reported for zebra fish (Rowe and Eckhert, 1999), frogs (Fort et al., 1998, 1999), and trout (Eckhert, 1998), and they suggest a function for boron in reproduction and development. However, boron-related developmental defects have not been found consistently in rodent models (Lanoue et al., 1998, 1999). Physiological effects, including changes in blood glucose and triglyceride concentrations and abnormal calcitriol (1,25,OH2D3) metabolism or function have been reported in boron-deficient chicks that have a concomitant vitamin D deficiency (Hunt, 1996). Higher insulin secretion from the pancreas of boron-deprived chicks has also been reported (Bakken, 1995). How-