[Na.sup.+]-to-sugar stoichiometry of SGLT3

[Na.sup.+]-to-sugar stoichiometry of SGLT3. Am J Physiol Renal Physiol 49: F278-F252, 2001.--Sodium-glucose cotransporters (SGLTs) mediate active transport of sugar across cell membranes coupled to [Na.sup.+], by using the electrochemical gradient as a driving force. In the kidney, there is evidence for two kinds of cotransporters, a high-affinity, low-capacity system, and a low-affinity, high-capacity system, with differences in substrate specificity and kinetics. Three renal SGLT clones have been identified: SGLT1 corresponding to the high-affinity system, and SGLT2 and SGLT3 with properties reminiscent of the low-affinity system. We have determined the stoichiometry of pig SGLT3 (pSGLT3) by using a direct method, comparing the substrate-induced inward charge to [sup.22]Na or [[sup.14]C][Alpha]-methyl--D-glucopyranoside uptake in the same oocyte, pSGLT3 stoichiometry is 2 [Na.sup.+]:1 sugar, the same as that for SGLT1, but different from SGLT2 (1:1). The [Na.sup.+] Hill coefficient for SGLT3 is ~1.5, suggesting low cooperativity between [Na.sup.+] binding sites. Thus SGLT3 has functional characteristics intermediate between SGLT1 and SGLT2, so, whereas SGLT3 stoichiometry is the same as that for SGLT1 (2:1), sugar affinity and specificity are similar to SGLT2. sodium-glucose cotransporter; electrophysiology; ion-substrate coupling