Facilitated diffusion of monosaccharides in Saccharomyces cerevisiae: experimental investigation of kinetic parameters without the assumptions of symmetry.

Until the question of symmetry or asymmetry in the facilitated diffusion of monosaccharides by Saccharomyces cerevisiae is resolved, attempts to study the transport process cannot be based on assumptions of symmetry, such as equal concentrations at equilibrium or kinetic parameters that are equal in opposite directions. The assumptions of symmetry may be circumvented by measuring efflux against water and against various external concentrations of sugar. The measurement of efflux against water eliminates any involvement of influx, and the separate determinations of influx and efflux parameters do not require that the parameters be equal. Furthermore, the use of relative internal concentrations eliminates any necessity of assuming that the equilibrium concentrations are equal. Since the influx and efflux parameters are to be compared, the measurement of influx on effluxing cells allows both sets of parameters to be determined on cells which are physiologically the same. This procedure has been tested by obtaining the kinetic parameters of l-sorbose transport. The validity of these parameters was demonstrated by using them to generate theoretical efflux curves that fit the experimental data and by showing that they give the best fit curve to the relationship of velocity and permeant concentration. Although the question of symmetry remains unanswered, this procedure has opened the way for experimental evaluation of the situation and further investigation of the transport process in yeast.