KINETIC-PARAMETERS OF MONOVALENT CATION UPTAKE IN YEAST CALCULATED ONACCOUNTING FOR THE MUTUAL INTERACTION OF CATION UPTAKE AND MEMBRANE-POTENTIAL

Kinetical parameters of monovalent cation uptake in yeast are calculat ed according to a model for mutual interaction of membrane potential a nd cation uptake. Apparent K(m) values for monovalent cation uptake ob tained from uptake studies are 3-5-times lower than the K(m) values ex pected when the membrane potential remains constant at increasing cati on concentrations instead of being reduced. The model accounts for var ious phenomena as (i) the increase in apparent K(m) of Rb+ uptake acco mpanying the decrease in maximum rate of uptake found on increasing th e cellular K+ content, (ii) the decrease in maximum uptake rate and in crease in K(m) in case of simultaneous transport of phosphate and Rb+, (iii) the change in the maximum uptake rate without a change in K(m) under conditions that the proton pump is effected and (iv) the absence of an increase in K+ efflux at high depolarizing external cation conc entrations.