Daniel I. Anion 329 



Space will not permit the mention of many other instances of enzyme 

 activation by metals which have come to light in recent years. Several 

 general conclusions, however, seem apparent. In some cases the activa- 

 tion appears to be specific for one element or at least it is greatly more 

 efficient with it than with any other. This was shown to be the case 

 for magnesium and manganese. Since both of these elements are 

 known to be essential for growth, the evidence of their importance in 

 specific enzyme reactions can be taken as an elucidation of their 

 metabolic function. In instances where several cations activate a given 

 system the evidence does not permit an unequivocal decision as to 

 which element is the actual activating agent in vivo. Nor indeed is 

 there any compelling reason to assume a priori that in the living cell, 

 reactions of this kind must always be catalyzed by one element only. 

 The situation becomes even more complex when work with isolated 

 enzymatic systems brings to light the activating properties of elements 

 such as nickel which are not known to be essential for life. 



It is possible that for certain cellular reactions specificity of the 

 activator is not a biological requirement. If this view is to be accepted, 

 how can it be reconciled with the well-established specificity of each 

 of the essential elements in growth experiments? It may be assumed 

 that among the several functions which an essential element performs 

 there is at least one for which it is specifically required, no substitution 

 being possible. This view would retain the concept of essentiality in 

 the sense that no sum of partial substitutions for individual reactions, 

 assuming that they all became known, could ever succeed in replacing 

 an essential element. It would, of course, be in accord with the known 

 facts for most of the essential elements. Magnesium, for example, acts 

 as a nonspecific activator of certain enzyme systems though it is also 

 a specific activator for others and no other element can take its place 

 as a component of chlorophyll. This hypothesis would also be compati- 

 ble with reports of "beneficial" effects on the plant from the addition of 

 nonessential elements. Sodium, for instance, could partly substitute 

 for potassium in the sense that it could take over, at least in part, one 

 of its functions. Beneficial effects from adding nonessential elements 

 would thus merely reflect suboptimal conditions with respect to the 

 supply of the essential elements. According to this view a nutrient 



