Three different mammalian myo-inositol cotransporters are currently known; two are the Na⁺-coupled SMIT1 and SMIT2 tabulated below and the third is proton-coupled HMIT (SLC2A13). SMIT1 and SMIT2 have a widespread and overlapping tissue location but in polarized cells, such as the Madin-Darby canine kidney cell line, they segregate to the basolateral and apical membranes, respectively [2]. In the nephron, SMIT1 mediates myo-inositol uptake as a ‘compatible osmolyte’ when inner medullary tubules are exposed to increases in extracellular osmolality, whilst SMIT2 mediates the reabsorption of myo-inositol from the filtrate. In some species (e.g. rat, but not rabbit) apically located SMIT2 is responsible for the uptake of myo-inositol from the intestinal lumen [1].

The data tabulated are those for dog SMIT1 and rabbit SMIT2. SMIT2 transports D-chiro-inositol, but SMIT1 does not. In addition, whereas SMIT1 transports both D-xylose and L-xylose and D-fucose and L-fucose, SMIT2 transports only the D-isomers of these sugars [4-5]. Thus the substrate specificities of SMIT1 (for L-fucose) and SMIT2 (for D-chiro-inositol) allow discrimination between the two SMITs. Human SMIT2 appears not to transport glucose [6].

1. Aouameur R, Da Cal S, Bissonnette P, Coady MJ, Lapointe JY. (2007) SMIT2 mediates all myo-inositol uptake in apical membranes of rat small intestine. Am J Physiol Gastrointest Liver Physiol, 293 (6): G1300-7. [PMID:17932225]

2. Bissonnette P, Coady MJ, Lapointe JY. (2004) Expression of the sodium-myo-inositol cotransporter SMIT2 at the apical membrane of Madin-Darby canine kidney cells. J Physiol (Lond.), 558 (Pt 3): 759-68. [PMID:15181167]

3. Bourgeois F, Coady MJ, Lapointe JY. (2005) Determination of transport stoichiometry for two cation-coupled myo-inositol cotransporters: SMIT2 and HMIT. J Physiol (Lond.), 563 (Pt 2): 333-43. [PMID:15613375]

4. Coady MJ, Wallendorff B, Gagnon DG, Lapointe JY. (2002) Identification of a novel Na+/myo-inositol cotransporter. J Biol Chem, 277 (38): 35219-24. [PMID:12133831]

5. Hager K, Hazama A, Kwon HM, Loo DD, Handler JS, Wright EM. (1995) Kinetics and specificity of the renal Na+/myo-inositol cotransporter expressed in Xenopus oocytes. J Membr Biol, 143 (2): 103-13. [PMID:7537337]

6. Lin X, Ma L, Fitzgerald RL, Ostlund RE. (2009) Human sodium/inositol cotransporter 2 (SMIT2) transports inositols but not glucose in L6 cells. Arch Biochem Biophys, 481 (2): 197-201. [PMID:19032932]