Hydrolysis of α-D-Glucopyranosyl-1, 6-Sorbitol and α-D-Glucopyranosyl-1, 6-Mannitol by Rat Intestinal Disaccharidases

Abstract

The hydrolyzing activities in rat small intestine for newly developed sugar substitutes, α-D-glucopyranosyl-l, 6-sorbitol (GPS) and α-D-glucopyranosyl-l, 6-mannitol (GPM), both of which are produced by hydrogenation of palatinose, were characterized. GPS and GPM were hydrolyzed in mucosal homogenate as well as in brush border membranes of rat small intestine at a slower rate than the rate of hydrolysis of palatinose (30%) and sucrose (6-7%). Gel filtration column chromatog-raphy of disaccharidases solubilized from brush border membranes revealed that GPS and GPM were hydrolyzed mainly by sucrase-isomaltase complex and its degradation product, i.e. isomaltase monomer. The isomaltase monomer, purified from small intestine of rats, possessed similar K_m values for GPS (2.47mM) and GPM (5.38mM) as compared to those of purified sucrase-isomaltase complex. The V_max values of isomal-tase monomer for GPS and GPM were twice as high as those of sucrase-isomaltase, suggesting that GPS and GPM are hydrolyzed by the active site of isomaltase. On the other hand, a small amount (up to 17%) of GPS-and GPM-hydrolyzing activities was ascribed to glucoamylase, which possessed relatively high Km values for GPS (18.7 mM) and GPM (32.9mM). To examine a physiological significance of GPS- and GPM-hydrolyzing activities, the transmural potential difference (ΔPD) evoked by Na⁺ -dependent active transport of glucose, produced by the hydrolysis of these disaccharide alcohols, was measured in everted segments of rat jejunum. The relative rates of absorption of glucose produced by the hydrolysis of GPS and GPM were 36% and 27% of that of palatinose, directly reflecting the hydrolyzing activities determined in jejunal ho-mogenate. These results suggest that the process of hydrolysis is the rate limiting step in digestion-absorption process of palatinose, GPS and GPM in small intestine.