STUDIES IN CARBOHYDRATE METABOLISM

Abstract

In an earlier paper in this series observations were reported on the rate at which glycogen in the liver and carcass of rats was being replaced by freshly synthesized glycogen while the animals were maintained at constant weight (1). The rats used in those experiments were fed a diet containing 60 per cent of glucose. From determination of the rate at which deuterium was incorporated into glycogen while the body fluids of the rats were enriched with DzO, it could be shown that the half life of glycogen was approximately 1.0 day in the liver and 3.6 days in the carcass, and that only about 0.44 gm. of freshly synthesized glycogen was deposited each day in the entire body of an adult female rat under these circumstances. The present report deals with similar experiments in which d-galactose was the sole dietary carbohydrate. Calactose was selected as the test sugar because earlier workers had found it to be a slow glycogen former (2), and also because, on structural grounds, it appeared doubtful whether galactose could be converted into glucose without prior rupture of the hexose chain into triose fragments. We have previously indicated the reasons for our expectation that glycogen, synthesized from fragments smaller than hexose in an animal whose body fluids contain DzO, should, when isolated, approach in isotope concentration 66 per cent of that in the body water (1). Experimentally, deuterium concentrations of 56 to 57 per cent of that in the body water were obtained in the liver glycogen in a short time when lactic acid (3) or alanine’ served as precursor. If, starting from galactose, glycogenesis proceeds exclusively through lactate, similarly high isotope concentrations might be anticipated in the glycogen deposited. Rats were maintained on a 60 per cent galactose diet for 1, 2, 4, and 8 days after the initiation of DzO administration, and the rise in deuterium concentration in the glycogen of liver and carcass was observed. From the analytical values obtained (Table I) it is clear that in both liver and