A model for metal selenide formation under biological conditions

The essential trace element selenium is known to be capable of reducing the toxicity of heavy metals like cadmium(II), mercury(II) and silver(I). One mechanism by which this occurs is through the formation of an inert metal selenide like silver selenide. It is not clear why selenide should react in the body preferentially with silver(I) over other metals such as zinc(II), an ion which is capable of forming zinc selenide, and which is present at relatively high levels. A reaction is discussed which can account for the ability of mercury(II) and silver(I) to lead to metal selenide formation in preference to zinc(II). Metal ions which have a sufficiently large formation constant for the metal selenide can induce the disproportionation of elemental selenium to produce the metal selenide; both mercury(II) and silver(I) can induce such reactions, whereas zinc(II) cannot. If this model is accurate, it should provide a basis to predict which metal ions can produce metal selenides in the body, and which ions are unlikely to result in metal selenide formation.