The Pentose Cycle, Triose Phosphate Isomerization, and Lipogenesis in Rat Adipose Tissue

Abstract

1. The relationship between the specific activities of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate from glucose-1-14C and -6-14C and triose-P isomerization has been analyzed. The specific activities are a function of three independent variables, the rate of triose-P isomerization, the contribution of the pentose cycle to glucose metabolism, and relative outflow from the dihydroxyacetone-P or glyceraldehyde-P pools. Equations to calculate the relative specific activities of these two compounds, the rate of triose-P isomerase, and the pentose cycle contribution under conditions of incomplete isotopic equilibration of triose-P have been derived. When synthesis of glycerol is limited, previously derived methods for evaluating the pentose cycle with the use of glucose-1-14C and -6-14C can be applied. 2. The contribution of the pentose cycle was calculated from 14C yields from glucose-1-14C and -6-14C, and from uniformly labeled 14C-glucose, and from degradation of glycerol derived from glucose-2-14C on incubation of epididymal fat tissue from rats. On the addition of epinephrine and growth hormone, similar estimates were obtained by three independent methods, theoretically valid in the absence of complete triose-P isomerization. Two methods, requiring complete equilibration, differed. Under other conditions, all five methods agreed closely. The contribution of the pentose cycle was correlated with fatty acid synthesis. In tissue from rats fasted and refed, or with insulin stimulation, the contribution averaged 25%. In the presence of growth hormone it averaged 11%. In rats fed ad libitum and without hormone, the contribution averaged 14%, and 7% in the presence of epinephrine. 3. Specific activities of the triose phosphates from glucose-1-14C and -6-14C and their rates of isomerization were estimated for the intact adipose tissue. The relative rates ranged from 2 to 10 times those of glucose utilization and were not greatly affected by dietary conditions or hormones. 4. Glucose utilization by tissue from rats fasted and refed, with insulin present, was seven times as high as that of tissue from rats fed ad libitum and without insulin. Of the utilized glucose carbon, 38% appeared in CO2, 45% in fatty acids, and 5% in glycerol. About two-thirds of the CO2 was derived via pyruvate decarboxylation and one-third via the pentose cycle; very little CO2 was evolved via the Krebs cycle. Epinephrine stimulated glucose utilization 3-fold. Of the glucose carbon, 30% appeared in CO2, 7% in fatty acids, and 40% in glycerol. Over half of the CO2 arose via the Krebs cycle, and one-tenth or less via the pentose cycle. Depending on diet and hormones, intermediate patterns between these two extremes were observed. 5. A balance of formation and utilization of reduced pyridine nucleotides in the cytoplasm was established. The total amount of reduced di- and triphosphopyridine-nucleotides formed in the cytoplasm was in small excess over hydrogen equivalents required for the synthesis of fatty acids, glycerol, and lactate. Under conditions of enhanced fatty acid synthesis, the TPNH formed via the pentose cycle was found to provide only 59 to 88% of the hydrogen required for fatty acid synthesis.