The Metabolism of Tritiated Glucose by Rat Adipose Tissue

From the ¹ From the Cedars-Sinai Medical Research Institute, Cedars-Sinai Medical Center, Los Angeles, California 90029

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 position of glycerol. Only trace amounts from this position were found in the position of lactate and the ß 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 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 and ß positions of glycerol. The T:¹⁴C ratio in the 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.

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