Simulation of differential effects on rates in membrane transport

Abstract

Solute uptake by a mobile carrier mechanism has been simulated with an analog computer for the case where the movement of the carrier across the membrane is rate-limiting. The ratio of the rate constants for the translocation of the loaded carrier to those for the translocation of the empty carrier was varied to simulate three important conditions of transport: a ratio greater than unity, which led as previously predicted to trans stimulation; a ratio less than unity, which led as predicted to trans inhibition; and a ratio equal to unity, which eliminated any effect of solute concentration on the trans flux. In addition, transients related to the relative amount of the mediator species are recorded. When previous loading of the cells with the solute was simulated, we obtained overshoots in the uptake of labeled solute corresponding to those often seen experimentally. The inhibitory action of 2,4-dinitrofluorobenzene on a well characterized laboratory example of trans stimulation was analyzed in terms of the model. Whereas the simulation shows that a change in the ratio of the rate constants for translocation can modify the intensity of the trans phenomenon, the action of dinitrofluorobenzene proved instead to arise from decreases in the two carrier fluxes that cause the transport in question to be uphill. The distribution of the influence between these two fluxes may assist identification of the mode of energization of the process.