A method to distinguish between pore and carrier kinetics applied to urea transport across the erythrocyte membrane

Permeability coefficients (P) measured at various penetrant concentrations (C) by the perturbation method can be plotted to distinguish simple diffusion, simple pore kinetics and simple carrier kinetics as follows: for simple diffusion, 1/P = constant; for a simple pore, 1/P = 1/Po + 1/Po[1/Kin + 1/Ko]C; for a simple carrier, 1/P = 1/Po + 1/Po[1/Kin + 1/Ko]C + 1/Po[1/(K3K4)] C2 where Po is the maximal permeability at zero penetrant concentration and the K's are combinations of kinetic constants defining each of the transport steps. (Kin and Ko are the half-saturation constant for zero-trans efflux and influx, respectively; K3 is the half-saturation constant for equilibrium exchange, and K4 is related to the mobility of the free carrier). In human erythrocytes, permeability coefficients for diethylene glycol were constant suggesting simple diffusion. For glucose, a plot of 1/P versus concentration was nonlinear indicating carrier kinetics. Plots of 1/P versus penetrant concentrations gave straight lines with positive slopes for urea in human and bovine erythrocytes and for methylurea in human red cells, indicating these penetrants follow simple pore kinetics or simple carrier kinetics in which K4 is very large.