Sugar Transport and Fructose Metabolism in Human Intestine In Vitro*

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Abstract
Sugar transport and fructose metabolism were studied in specimens of human intestinal mucosa removed at surgery. Transport of sugars in man conforms to structural requirements in the hamster and some sugars are transported actively as shown by intracellular accumulation, whereas other sugars, such as fructose, are not. Fructose is converted to glucose as measured by incorporation of C14 from fructose-1-C14 and fructose-6-C14 into glucose and glycogen. Initial metabolism of fructose is primarily via fructose-1-phosphate, as evidenced by the labeling pattern obtained with fructose-1-C14 and fructose-6-C14 and by the hydrolysis characteristic of the phosphate formed from fructose. Glucose-6-phosphatase activity is present, as evidenced by the preferential hydrolysis of glucose-6-phosphate over glucose-1-phos-phate at acid pH. The pentose cycle is active in human intestine, as measured by randomization of glucose-2-C14 and the transaldolise-eatalyzed reactions, which are associated with the cycle, play a part in the incorporation of C14 from fructose-C14 into glucose. Glyceraldehyde is initially metabolized via glyceraldehyde-3-phosphate, as determined by the labeling pattern obtained with glyceraldehyde-3-C14.