ABSTRACT
I. INTRODUCTION Selenium, discovered in 1817 by Berzelius and named after the Greek goddess of the moon Selene, is a rare element belonging to main group VI in the periodic table of elements. As a typical chalcogen it shares many properties with its closest relative sulfur. Similar to sulfur it occurs in the oxidation states −2, 0, +2, +4, and +6 (Table 1), which implies that it is prone to release or accept pairs of electrons. Selenium compounds with an uneven number of electrons (i.e., stable selenium radicals) are unknown. Short-lived selenium radicals may, however, be formed. As has also been demonstrated for sulfur amino acids (1,2), complexation of selenols with transition metals may facilitate single-electron transitions resulting in the generation of superoxide in the presence of oxygen. Formation of superoxide, H2O2, and OH radicals in aerobic solution of H2Se or mixtures of selenite and thiols, which lead to the formation of selenide, have indeed been detected (3,4). Inversely, synthetic benzselenazol derivatives, such as ebselen (PZ51), may react with free radicals and, therefore, were discussed as “antioxidants” (5), but the selenium radical thereby produced proved to be reactive enough to propagate a chain reaction instead of terminating it (6). So far, no selenium compound has fulfilled the requirement of a real antioxidant, which in its proper definition, is an element or compound that scavenges radicals, is itself transformed into a rather inert radical, and thereby terminates a radical-driven chain reaction (7); nor can selenium be generally rated as an antioxidant in the broader sense of creating a reductive environment in a biological system. Selenate is a stronger oxidant than sulfate; elemental selenium is quite inert. Naturally occurring compounds with selenium, present in low-oxidation states, are reductants, but may generate strong oxidants by reacting with the most abundant oxidant, molecular oxygen (3,4). Similarly, some synthetic
Table 1 Oxidation States of Selenium
Chemical structure Oxidation state Name of acid Name of salts
H2Se −II Hydrogen selenide Selenide R-SeH −II Selenol Se 0 Selenium R-Se-OH 0 Selenenic acid Selenenate
O‖ R-Se-OH +II Seleninic acid Seleninate
O‖ HO-Se-OH +IV Selenious acid Selenite (SeO32−)
O‖ HO-Se-OH +VI Selenic acid Selenate (SeO42−)‖
O
compounds related to ebselen even proved to be pro-oxidants, whereas others, including ebselen itself, reduced peroxides and thus mimicked glutathione peroxidase (8).
