The concentration of selenium in the Earth's crust is less than that of gold. The chemical and physical properties of selenium are very similar to those of sulfur.1 Selenium and sulfur have similar outer-valence shell electronic configurations and atomic sizes and their bond energies, ionization potentials, and electron affinities are virtually the same. Despite these similarities, the biochemistry of selenium and sulfur differ in at least two respects that distinguish them in biological systems. First, selenium compounds are metabolized to more reduced states, whereas sulfur compounds are metabolized to more oxidized states. Second, these compounds differ in the acid strengths of their hydrides. The hydride, H2Se, is much more acidic than is H2S. This difference in acidic strengths is reflected in the dissociation behaviors of the selenohydryl groups of selenocys-teine and the sulfhydryl groups on cysteine. Hence, while thiols such as cysteine are predominantly protonated at physiological pHs, the selenohydryl groups of selenols such as selenocysteine are predominantly dissociated under the same conditions. These chemical differences between selenium and sulfur are the reasons selenocompounds are usually 600 times more effective than their sulfur analogs against tumors.2

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