An experimental test for cyclic versus linear transport models. The mechanisms of glucose and choline transport in erythrocytes.

Numerous mechanisms have been suggested to explain transport across biological membranes, all of which fall into one or the other of two distinct categories. In some, substrate sites in the free carrier are simultaneously exposed on the two membrane surfaces, while in others a substrate site is alternately exposed on opposite sides. Either group could account for active and facilitated transport, as well as for accelerated exchange, countertransport, and hyperbolic substrate saturation curves. A simple kinetic test is described here which distinguishes between these two classes. The test depends on measurements of transport rates in the presence of reversible competitive inhibitors inside and outside the cell. Experiments are reported on the glucose system of erythrocytes involving the inhibitors phloretin and cytochalasin B, and on the choline system of the same cells, with the nontransported substrate analogs dimethyl-n-pentyl(2-hydroxyethyl)ammonium ion and 2-di-butylaminoethanol. The results are in agreement with the single site-exposure models, which include the classical carrier, and incompatible with the dual site-exposure models. The mechanisms in the latter group are therefore rejected as explanations for glucose or choline transport.