Studies on the Metabolism of Adipose Tissue

Rat epididymal fat pads were incubated in vitro at 37° in bicarbonate buffer, pH 7.4, with uniformly labeled pyruvate-14C. The net gas exchange, total CO2 production, O2 consumption, pyruvate utilization, lactate and glycerol production, and radioactive carbon incorporation into CO2, fatty acids, and glyceride glycerol were measured. The results were used to estimate the quantitative flow of pyruvate into its major pathways of metabolism in adipose tissue. An exchange reaction between the CO2 phase of the medium and pyruvate occurs, which results in 14CO2 production from pyruvate-14C over and above that produced as a result of oxidation of the pyruvate. The extent of this exchange reaction was measured by incubation of tissue with unlabeled pyruvate and NaH14CO3. Radioactivity appeared in the medium in a product, presumably pyruvate, which was precipitated quantitatively as the 2,4-dinitrophenylhydrazone derivative. Radioactivity was also found in glyceride glycerol, but not in fatty acids. Radioactive fatty acid formation was also measured from mixtures of pyruvate-14C with unlabeled acetate or lactate, and from unlabeled pyruvate and acetate-1,2-14C or lactate-2,314C. The results permit a calculation to be made of the total reduced coenzymes produced and utilized during the metabolism of pyruvate. Up to 30% of the reduced coenzymes formed by the oxidation of pyruvate may be utilized in reduction reactions that are generally accepted to occur in the extramitochondrial space of the cell. If it is assumed that production of reduced coenzymes from pyruvate occurs entirely within the mitochondria, then there arises the question of the form in which reducing equivalents are transported across the mitochondrial membrane. This question is discussed in relation to enzymatic reactions known to occur in adipose tissue and to the increased oxygen consumption that accompanies the conversion of pyruvate to fatty acids.