Proton-linked monocarboxylate transporters (MCTs) are a family of highly homologous trans-membrane symporters, involved in the transportation of monocarboxylic acids across the plasma membrane such as: lactate, pyruvate, branched-chain oxoacids derived from leucine, valine, and isoleucine, ketone bodies, acetoacetate, β-hydroxybutyrate and acetate. Thus, MCTs play an important role in metabolic communication among cells and in mammalian metabolism.

MCT3 is encoded by the Slc16a8 gene and contains at least nine members in humans¹. Structurally, MCTs have 12 transmembrane spanning domains with cytoplasmic NH₂- and COOH-terminals. The COOH-terminal regions are not well conserved among the MCTs isoforms while the membrane spanning domains share the greatest sequence identity^3,4. Findings suggest that the N-terminal domain is more important for energy coupling, membrane insertion and correct structure maintenance, whereas the C-terminal domain is more important for the determination of substrate specificity¹.

MCT3 is expressed in the retinal pigment epithelium, preferentially expressed in the basolateral membrane in the adult and in choroid plexus epithelium where it plays a role in regulating pH and lactate concentrations in the outer retina. Reduced levels of MCT3 in response to trauma or disease could contribute to pathologic changes in the retina^2,3. MCT3 can form a heteromeric complex with the accessory protein CD147, an immunoglobulin superfamily protein.