Time course analysis of cotransport in membrane vesicles: solutes and tracers

A theoretical analysis of the time course of a ternary cotransport system in membrane vesicles has been developed by extending previous work (Weiss, S.D. et al. (1981) J. Theor. Biol. 93, 597-608; Heinz, E. and Weinstein, M. (1984) Biochim. Biophys. Acta 776, 83-91). It has been assumed that the translocation of the carrier is the rate-limiting step of the transport process. Our approach includes, in particular, the presence of isotope tracer fluxes and the generalization to the case when many solutes share the same carrier. The situation when the tracer and the solute behave differently, as in the countertransport case, is stressed. Also, the interaction of two different solutes, internal and external to vesicles, is considered. Other points regard the analysis of the solute binding to the membrane vesicles, the influence of water permeability and the possible asymmetry of the transport system. In the Appendix, the assumption of no net translocation of all carrier species is discussed.