Interdependencies between metal ions

Metal ion toxicity does not relate to essentiality. Toxicity and essentiality do share, however, one feature frequent interdependencies among metal ions, between metal ions and nonmetals, and between both, and organic molecules such as vitamins. For example, excess Zn + may reduce absorption of essential Cu +, while a deficiency of Zn + worsens Cd + toxicity. Such antagonisms and mutual dependencies produce variable toxicities for like concentrations of an element. 

This discovery of Bollum (11) makes obsolete a number of previous excellent studies including those on mutual interdependence between concentrations of divalent metal, monovalent metal, hydrogen ion, and substrate. Unless the bond affected by the metal in question is specified, an overall rate represents a number with little value. The problem is further complicated by the suspected (by analogy to other nucleases) quantitative changes in requirements for metal ions at different stages of the reaction. So far no such data are available for DNase I. One is tempted to add, luckily, because in view of the uncertainty of qualitative effects such data would hardly be expected to have a long survival time. 

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