Stereoselective, SH-dependent transfer of lactate in mammalian erythrocytes

Abstract

The equilibrium exchange of lactate and glycolate was studied in human and other mammalian erythrocytes. Both anions penetrate by a process exhibiting Michaelis-Menten kinetics, slower than chloride but faster than sulfate. L-Lactate exchanges fater than d-lactate due to a different half saturation constant ( l-lactate 66 mM, d-lactate 357 mM). Maximal transfer rates are identical for both isomers. The extent of stereoselectivity is species-dependent. Lactate and glycolate transfer share basic characteristics with inorganic anion transfer (anion dependency, sensitivity to amphiphilic inhibitors (e.g. salicylate), to aminoreagents and pronase) but differ in others (pH dependency, low sensitivity to disulfonate inhibitors (4-acetamido-4′-isothiocyano stilbene-disulfonate, tetrathionate)). Activation energies are high (30–36 kcal/mol) at low temperature, but decrease (to 15–26 kcal) at higher temperature, breaks occurring between 10 and 20°C. p]In marked contrast to inorganic anion transfer, lactate and glycolate transfer are inhibited reversibly by impermeable and permeable SH-reagents of the mercurial (HgCl 2, p- chlormercuribenzene sulfonate (PCMBS)) and the dithiol (4,4′-dithiodipyridine, 5,5′-dithiobis(2-nitrobenzoate)) type. The inhibition is incomplete, reversible and varies in its extent from anion to anion. PCMBS acts instantaneously, indicating a very superficial localization of the SH-group involved. The results suggest that lactate and glycolate penetrate the erythrocyte membrane predominantly, although not exclusively, via an anion transfer system specialized for this type of anion, and different from the clasical inorganic anion exchange system.