Fructose Phosphorylation and Dephosphorylation by the Intestinal Mucosa of the Rat During Fructose Absorption

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Abstract
The role of phosphorylation in the absorption of fructose from the intestinal tract of the fasted rat by in vitro and in vivo techniques was studied. The authors' method for the determination of fructose phosphate esters was used and these esters were identified by paper chromatography and copper reduction techniques. Buffered homogenate of intestinal mucosa of a fasted rat, mixed with ATP, MgCl2, KF and fructose, when incubated at 30°, showed the formation of fructose-6-phosphate and fructose-1, 6-diphosphate at a rate that corresponded to the decrease in free fructose. The same homogenate, mixed with fructose-1, 6-diphosphate and incubated at 37°, showed the formation of fructose-6-phosphate and free fructose at a rate that corresponded to the decrease in the concentration of the diphosphate ester. Following intraduodenal injection of fructose solution into anesthetized fasted rats, homogenates of the intestinal mucosa showed the presence of fructose-6-phosphate and fructose-1, 6-diphosphate in average concentrations 14 and 5 times, respectively, those found in control muocsa, also concentrations of free fructose in the blood of the portal vein up to 24.6 mg % were observed. The large increase in fructose phosphate esters in the intestinal mucosa, observed after fructose administration, suggests that phosphorylation of sugars in absorption serves a more extensive function than to initiate glycolysis for the normal metabolism of the mucosal cells. The data obtained suggest that phosphorylation and dephosphorylation are functional steps in the absorption of fructose from the alimentary tract of the rat.