(1) Background: Human aquaporin-9 (AQP9) conducts several small uncharged metabolites, such as glycerol, urea, and lactic acid. Certain brain tumors were shown to upregulate AQP9 expression, and the putative increase in lactic acid permeability was assigned to severity. (2) Methods: We expressed AQP9 and human monocarboxylate transporter 1 (MCT1) in yeast to determine the uptake rates and accumulation of radiolabeled l-lactate/l-lactic acid in different external pH conditions. (3) Results: The AQP9-mediated uptake of l-lactic acid was slow compared to MCT1 at neutral and slightly acidic pH, due to low concentrations of the neutral substrate species. At a pH corresponding to the pKa of l-lactic acid, uptake via AQP9 was faster than via MCT1. Substrate accumulation was fundamentally different between AQP9 and MCT1. With MCT1, an equilibrium was reached, at which the intracellular and extracellular l-lactate/H+ concentrations were balanced. Uptake via AQP9 was linear, theoretically yielding orders of magnitude of higher substrate accumulation than MCT1. (4) Conclusions: The selectivity of AQP9 for neutral l-lactic acid establishes an ion trap for l-lactate after dissociation. This may be physiologically relevant if the transmembrane proton gradient is steep, and AQP9 acts as the sole uptake path on at least one side of a polarized cell.
Keywords: accumulation; aquaglyceroporin; gradient; ion trap; l-lactate; l-lactic acid; monocarboxylate transporter; permeability; proton; transport.