Proteins in bioelectricity: the control of ion movements across excitable membranes.

Investigations of the last decade have revealed that cell membranes, although only 100 A thick, are the site of many complex enzyme reactions, as illustrated, for example, by the well-studied mitochondrial membranes (see refs. 1-3). The molecular organization of membranes is far from being established; they are apparently a mosaic of functional units formed by lipoprotein complexes.4 The excitable membranes, i.e. the plasma membranes surrounding nerve and muscle fibers, have the special ability to change, rapidly and reversibly, their permeability to ions. The resulting ion movements are the carriers of bioelectric currents propagating nerve impulses along nerve and muscle fibers. The idea of a purely physical process assumed in the ionic theory is difficult to reconcile with the large amounts of heat produced and absorbed during electrical activity. The most likely explanation of the results of the heat measurements is the assumption that chemical reactions underly the permeability cycle in excitable membranes.5 In view of the central role of enzymes and proteins in cell mechanisms, it appears reasonable to assume that they play an essential role in the elementary process of excitable membranes, i.e. in bioelectricity. In the last 30 years evidence has accumulated for the assumption that acetylcholine (ACh) acts as a trigger, controlling ion movements across excitable membranes. The principal bases for this view may be briefly summarized: (1) ACh and the enzymes which hydrolyze and form it (ACh-esterase and choline O-acetyltransferase (choline acetylase)) have been shown to be present in all types of conducting fibers of nerve and muscle throughout the animal kingdom: In motor and sensory, "cholinergic" and "adrenergic," peripheral and central fibers, in invertebrates and vertebrates, etc. (2) ACh-esterase, an enzyme relatively specific for ACh and distinctly different from other esterases, is localized in the excitable membranes of axons and muscle fibers as well as in those of junctions, i.e. in the membranes of nerve terminals and in the postsynaptic membranes. (3) ACh-esterase hydrolyzes ACh in a few microseconds, a prerequisite for attributing to the ester the proposed role as a trigger in the generation of bioelectric currents. (4) An extraordinarily high concentration of the enzyme has been found in the electric organs of electric fish. These organs, the most powerful bioelectric generators developed by nature, are highly specialized in their function. Although formed by 3 per cent protein and 92 per cent water, 1 kg (fresh weight) of electric tissue of Torpedo and Electrophorus hydrolyzes 3-4 kg of ACh per hour. For the last 30 years this tissue has been essential for the analysis of the proteins specifically associated with bioelectrogenesis. (5) Specific and potent inhibitors of ACh-esterase have been shown to block electrical activity in a great variety of nerve fibers, e.g. reversibly with physostigmine and irreversibly with diisopropylphosphofluoridate (DFP). Thus, electrical activity requires the