Trace elements and vitamins in membrane function.

The title of this article might well have sprung directly from an examination question, for example ‘Discuss the role of trace elements and vitamins in membrane function’. One could imagine perplexed students, faced with this enigmatic problem, racking their memories for some clue to the thoughts of a perverse examiner. For of course, the question is meaningless. There is no systematic reason why any particular link should be drawn between membrane function and trace elements or vitamins. Nevertheless, this question does raise a more general one for descriptive biology. What are the characteristics possessed by that assorted and disparate group of inorganic and organic molecules, the so-called vitamins and trace elements, which justlfy their collective classification as micronutrients? It is clear that they are environmental molecules required in ‘small’ amounts for specific biochemical functions. However, another characteristic they have in common is the fact that their functions, unlike those of most macronutrients, each depend on a particular intrinsic chemical property which is maintained in vivo but which can be readily demonstrated in simple, non-biological systems. In contrast the metabolic substrates, the macronutrients, provide the atoms for structural use as well as the energy from chemical bonds to power biochemical processing. The functional derivatives of the macronutrients can thus be very different from the actual food molecules absorbed by the gut. Trace elements and vitamins are chemically-active substances having biological functions defined by their chemical reactivity. They are concerned with the management of biochemistry rather than being the substrates of biochemical reactions. As an example of this utilization of a specific chemical property in biology one can consider the micronutrient, pyridoxal phosphate. In vitro, the aldehyde, group of this molecule forms a Schiff-base bond with the amino group of an amino acid. In vivo, when pyridoxal phosphate is associated with specific enzyme proteins, such Schiff-base complexes allow catalytic transamination or decarboxylation of amino acids to occur. It is of interest when assessing the chemical roles of trace elements and vitamins in biology, to speculate about the evolutionary adoption of these molecules as essential components of biochemistry. Apart from the loss of preexisting biosyntheses of these substances, there must have been stages in evolutionary development when biological systems benefitted from the incorporation of environmental molecules with particular chemical functions. Such a consideration might help to identify a feature of some micronutrients which could apply to the present topic: a relation to membrane function.