The Metabolism of Tritiated Glucose by Rat Adipose Tissue

Abstract

Glucose, labeled with 14C uniformly or in positions 1 or 6, and with tritium in positions 1, 3, 4, or 6, was incubated with rat epididymal fat tissue in the presence of insulin or epinephrine. Incorporation of the label into CO2, fatty acids, glycerol, lactate, and water was determined. From the 14C yields, the contribution of the pentose cycle to glucose metabolism was calculated. A carbon balance and a balance of formation and utilization of reduced pyridine nucleotides in the cytoplasm were established. Tritium from glucose-3-T was recovered in water and fatty acids, and a little in the α position of glycerol. Only trace amounts from this position were found in the α position of lactate and the β position of glycerol. The fatty acid yield of tritium from this sugar equaled the 14CO2 yield from glucose-1-14C via the pentose cycle. The yield of fatty acids from glucose-3-T was twice that of the yield of fatty acids via tritium-labeled reduced triphosphopyridine nucleotide from glucose-1-T. In glycerol formed from glucose-1- and -6-T, all the tritium was recovered in the α position. In the presence of insulin, the T:14C ratio in glycerol from glucose-6-T ,6-14C was about 1.0; that of glycerol from glucose-1-T ,1-14C was 1.2 to 1.3. The T:14C ratios in lactate from these two sugars were less than 1, but the T:14C ratio from glucose-1-T ,1-14C was about 1.2 times that from glucose-6-T ,6-14C. In the presence of epinephrine the ratios in glycerol and lactate were the same for these two labeled sugars. Nearly 90% of tritium from glucose-4-T appeared in water in the presence of insulin, and 60% in the presence of epinephrine. The rest was recovered in lactate, fatty acids, and the α and β positions of glycerol. The T:14C ratio in the α position of glycerol from glucose-4-T labeled uniformly with 14C indicated extensive loss of the tritium from this sugar prior to the oxidation of triose phosphate. The T:14C ratios of position 2 of glycerol and of lactate indicated further loss of tritium. The contribution of TPNH and DPNH to fatty acid synthesis was calculated independently from 14C and tritium data. In the presence of insulin, it appears from 14C data that with rats on high carbohydrate diet 50% of the hydrogen equivalents are provided by TPNH, and that with rats on a regular diet 75% are provided by TPNH. From tritium data it appears that, with both diets, about 75% of the hydrogen equivalents are provided by TPNH and 25% by DPNH. With epinephrine, when fatty acid synthesis is low, DPNH may be the major source of reducing hydrogen.