Slip and leak models of gradient-coupled solute transport.

There is now widespread support for the general concept that the accumulation of certain solutes in various prokaryotic and eukaryotic systems is energized by the circulation of protons or of Nat ions across the plasmalemma. The way in which energy coupling occurs, however, is poorly understood, progress being hindered both by the present lack of information about its molecular basis and by the complexity of plausible kinetic models of such systems. A number of studies of these symports have shown that, provided that solute metabolism is an unimportant factor, a steady state is achieved in which the ratio (cellular solute concentration)/(extracellular solute concentration) ([Sl,/[Sl,) increases with time to a constant value. This ratio decreases markedly as the initial extracellular solute concentration is raised, as though the flow of solute becomes uncoupled from its energy supply when [Sl, is large. Indeed, as IS], rises, the concentration of solute in the cellular pool, the 'pool size', may appear to approach a maximum value. The outcome is that lSl,/[Sl, may vary with the solute concentration by more than a factor of lo2 in the case of a-thioethyl glucoside accumulation by yeast (Eddy et al., 1979) and by at least a factor of 10 in the systems of the mouse ascites-tumour cells that concentrate amino acids (Scheme 2). The purpose of the present essay is twofold: on the one hand, to consider some of the factors involved in the regulation of carbohydrate and amino acid accumulation in these eukaryotic systems, and, on the other hand, to relate the concentration-dependence of the ratio [Sl,/[Sl, to the mechanism of energy coupling. The regulation of solute accumulation can be considered in terms of the so-called 'coarse' and 'fine' controls that regulate enzyme activity. For instance, the repression of the general amino acid permease of yeast during growth in the presence of NH,+ and its de-repression during a subsequent period of nitrogen starvation represents a coarse control. A superficially analogous system may control amino acid accumulation in certain mammalian cells (Guidotti et al., 1978). A fine control on the activity of the yeast amino acid permease is exerted by amino acids that have already accumulated in the cells. The limited evidence available suggests that these initiate the inactivation of the permease in a way that both reflects the pool size and eventually limits the amount of amino acid absorbed. Thus failure of the mechanism can lead to the osmotic explosion of the cells, a dramatic demonstration of the magnitude of the power input into the amino acid pump (Indge et al., 1977). Glycine uptake through the yeast general permease leads to the simultaneous absorption of 2 equiv. of protons down a proton gradient estimated at about 0.3 V equivalent (Eddy, 1977). The largest glycine gradients produced by the yeast correspond to a factor of about 4 x lo4. Clearly such ratios are very much smaller than correspond to thermodynamic equilibrium with the proton gradient if the system transfers two protons with each glycine molecule traversing the cell membrane.