The Metabolism of Glucose-2-T by Adipose Tissue

Rat epididymal fat pad segments were incubated with glucose labeled uniformly with 14C, and with tritium in position 2, and in positions 1,3, and 6. Under conditions of high lipogenesis, 70 to 80% of tritium (T) from position 2 was recovered in water, with the remainder in lipid glycerol, fatty acids, and lactate. The highest tritium retention, relative to carbon, was in glycerol, with T:14C ratios from glucose uniformly labeled with 14C and with tritium in position 2 in the presence of insulin of 0.6 to 0.8. Of this tritium, 12 to 15% was on C-1 of glycerol. In the presence of epinephrine, when the pentose cycle was low, retention of tritium in position 2 was decreased. Retention and distribution of tritium from position 2 of glucose in hexose 6-phosphates is a function of four factors: (a) the rate of hexose 6-phosphate isomerase reaction; (b) the ratio of exchange to transfer of hydrogen in this reaction; (c) isotope discrimination against tritium in the isomerization; and (d) the contribution of the pentose cycle to glucose metabolism. Attempts to evaluate some of these parameters in adipose tissue are presented. It was estimated that about half of the hydrogen from position 2 of glucose 6-phosphate was exchanged with protons via the phosphohexose isomerase reaction. The observed high retention of tritium reflects the high level of pentose cycle in this tissue, isotope discrimination effects, and relatively slow rate of hexose 6-phosphate isomerase.