Monocarboxylates such as pyruvate, lactate, and ketone bodies play essential roles in carbohydrate, fat, and amino acid metabolism and must be rapidly transported across the plasma membrane of cells. Transport of monocarboxylates is mediated by proton-linked monocarboxylate transporters (MCTs).

MCT1 is comprised of 12-transmembrane helices (TMs) with intracellular C- and N-termini and a large cytosolic loop between TMs 6 and 7. The TM regions are more conserved than the loops and C-terminus. MCT1 demonstrates Michaelis Menten kinetics with a broad specificity for short-chain monocarboxylates such as halides, hydroxyl, and carbonyl groups. In addition, the transport of unsubstituted short-chain fatty acids, such as acetate, propionate, and butyrate, is strongly facilitated by MCT1. Natural occurring substrates for MCT1 include L-lactate, pyruvate, β-hydroxybutyrate, and acetoacetate. The predominant role of MCT1 is to facilitate unidirectional proton-linked transport of L-lactate across the plasma membrane. This may result in either influx or efflux of lactic acid depending of the prevailing intracellular and extracellular substrate concentrations and the pH gradient across the plasma membrane. When transporting lactate into the cell, MCT1 has a substrate binding site open to the extracellular matrix which binds a proton first followed by the lactate anion¹.

MCT1 has been considered as a promising target for cancer therapy, since it facilitates lactate efflux in glycolytic tumors. In breast carcinoma, MCT1 is associated with poor prognostic variables such as basal-like subtype and high grade tumors².