Kinetic analysis of L-lactate transport in human erythrocytes via the monocarboxylate-specific carrier system.

Three parallel pathways of L-lactate transport across the membrane of human red blood cells can be discriminated: (a) by nonionic diffusion; (b) via the band 3 anion exchange protein; and (c) via a specific monocarboxylate carrier system. Influx of lactate via the latter system leads to alkalinization of the medium, suggesting lactate-proton symport. Kinetic analysis of initial lactate influx via the monocarboxylate carrier indicates a symport system with ordered binding of the two ligands, in the sense that a proton binds first to the translocator, followed by lactate binding to the protonated carrier. The influence of varying trans-pH under conditions of net (zero-trans) flux with constant cis-pH indicates that the monocarboxylate translocator should be considered as a mobile carrier, with the ligand-binding sites exposed alternatively to the outside and the inside of the membrane.