The transport of L-leucine in human erythrocytes: a new kinetic analysis.

1. Data on the transport of L-leucine into human erythrocytes at 25 degrees C shows (a) that the carrier re-orientation process is rate determining, (b) that the binding of leucine to the carrier increases the rate of carrier re-orientation and (c) that the carrier is, at equilibrium, about equally distributed between the membrane surfaces at 25 degrees C.2. These conclusions are reached through a new kinetic analysis of a simple carrier system, which involves no prior assumptions about the relative magnitudes of the rate constants, yet leads to usable kinetic equations. These equations allow the determination of the rate determining step, the calculation of the effect of bound substrate on the rate of carrier re-orientation and, in some cases, an estimate to be made of the equilibrium distribution of the carrier between the inner and outer membrane surfaces.