Intestinal transport of β-thioglycosides by Na+/glucose cotransporter

Intestinal metabolism and transport of p-nitrophenyl beta-D-thioglucoside (p-NP beta Sglc) and p-nitrophenyl beta-D-thiogalactoside (p-NP beta Sgal) by the Na+/glucose cotransporter were studied in excised small intestine of the rat. p-NP beta Sglc and p-NP beta Sgal were stable enough on the mucosal side to be transported to the serosal side. Transport of p-NP beta Sglc was inhibited in the presence of phloridzin (a Na+/glucose cotransporter inhibitor), glucose, or 3-O-methylglucose (3-OMG). p-NP beta Sglc transport was dependent on Na concentration in a sigmoidal manner. The activation energy for transport was 19.4 kcal mol(-1). The distribution of transport activity of p-NP beta Sglc in each region of the small intestine correlated well with that of 3-OMG. These results indicate that p-NP beta Sglc is transported by the Na+/glucose cotransporter in small intestine. The order of turnover rate for glycoside transport by Na+/glucose cotransporter was 3-OMG > p-nitrophenyl beta-O-glucoside > p-NP beta Sglc > p-NP beta Sgal, indicating that the presence of a galactose moiety and a sulphur between the monosaccharide moiety and aglycone decreases the turnover rate of the Na+/glucose cotransporter in the transport of glycosides. In an inhibition study using stable p-NP beta Sglc as a Na+/glucose cotransporter-transportable marker glucoside, it was also shown that the Na+/glucose cotransporter recognized several types of glycosides. In conclusion, displacement of the oxygen at carbon C-1 of glucose by sulphur in thioglycosides decreases the turnover rate of the Na+/glucose cotransporter, but thioglycosides are stable in the small intestine and are transported by the Na+/glucose cotransporter.