Identification of a novel Na+/myo-inositol cotransporter

rkST1, an orphan cDNA of the SLC5 family (43% identical in sequence to the sodium myo-inositol cotransporter SMIT), was expressed in Xenopus laevis oocytes that were subsequently voltage-clamped and exposed to likely substrates. Whereas superfusion with glucose and other sugars produced a small inward current, the largest current was observed with myo-inositol. The expressed protein, which we have named SMIT2, cotransports myo-inositol with a K-m of 120 muM and displays a current-voltage relationship similar to that seen with SMIT (now called SMIT1). The transport is Na+-dependent, with a K-m of 13 mM. SMIT2 exhibits phlorizin-inhibitable presteady-state currents and substrate-independent "Na+ leak" currents similar to those of related cotransporters. The steady-state cotransport current is also phlorizin-inhibitable with a K-i of 76 muM. SMIT2 exhibits stereospecific cotransport of both D-glucose and D-xylose but does not transport fucose. In addition, SMIT2 (but not SMIT1) transports D-chiro-inositol. Based on previous publications, the tissue distribution of SMIT2 is different from that of SMIT1, and the existence of this second cotransporter may explain much of the heterogeneity that has been reported for inositol transport.