Sodium-coupled monocarboxylate transporter 2

Sodium-coupled monocarboxylate transporter 2 (i.e., SMCT2, also termed SLC5A12) is a plasma membrane transport protein in the solute carrier family. It transports sodium cations (i.e., Na⁺) in association with the anionic forms (see conjugated base) of certain short-chain fatty acids (i.e., SC-FAs) and other agents through the plasma membrane from the outside to the inside of cells. The only other member of the sodium-coupled monocarboxylate transporter group (sometimes referred to as the SLC5A family), SMCT1, similarly co-transports SC-FAs and other agents into cells. Monocarboxylate transporters (MCTs) are also transport proteins in the solute carrier family. They co-transport the anionic forms of various compounds into cells in association with hydrogen cations (i.e. H⁺). Four of the 14 MCTs, i.e. SLC16A1 (i.e., MCT1), SLC16A7 (i.e., MCT22), SLC16A8 (i.e., MCT3), and SLC16A3 (i.e., MCT4), transport some of the same SC-FAs anions that the two SMCTs transport into cells. SC-FAs do diffuse into cells independently of transport proteins but at the levels normally occurring in tissues greater amounts of the SC-FAs are brought into cells that express a SC-FA transporter.

The human gene responsible for producing SMCT2 protein, i.e., the SLC5A12 gene, is located at position 14.2 on the "p" (i.e., short) arm of chromosome 11 (position notated as chromosome 11p14.2). The full length SMCT2 protein product of this gene consist of 618 amino acids and has a 57% identity with the SMCT1 protein at the amino acid level. The gene for SMCT2 in mice and rats is termed the Slc5a12 gene.

Studies indicate that the SMCT2 on intestinal epithelial cells promotes their uptake of intra-intestinal SC-FAs and subsequent diffusion of the SC-FAs into the systemic circulation. These SC-FAs serve as energy sources for and activators of diverse responses in a wide range of cell types in the intestinal wall and throughout the entire body. Studies also suggest that: 1) SMCT2 in the kidney tubule cells contributes to the reabsorption of urinary SC-FAs that would otherwise be wasted in the urine; 2) the SMCT2 on the Müller cells in the eye promotes their uptake SC-FAs which they pass on to retinal neurons that use them as energy sources; and 3) the SCMT2 on skeletal muscle cells may contribute to regulating their lactic acid levels.