A high rate of anaerobic and aerobic glycolysis is the only biochemical feature which is common to all malignant tissues. The transformation of glucose into lactic acid in tumors undoubtedly takes place (as in normal cells) by the Embden-Meyerhof glycolytic sequence, with the concomitant synthesis of ATP. It has been reported that the levels of some respiratory coenzymes are often rather low in malignant tumors, and the extent to which respiration contributes to the formation of ATP in cancers is one of the most fundamental problems of tumor biochemistry. In 1951, at a time when very little was known about the respiratory machinery of cancer cells, mitochondria were isolated from a number of transplantable and highly malignant tumors of rodents, and the respiratory and phosphoryla-tive activities of these particles were investigated (200). It was found that the tumor mitochondria catalyzed the oxidation of a number of tricarboxylic acid cycle substrates. The tumor mitochondria appeared to be more fragile in comparison with similar particles isolated by the same methods from tissues such as liver and kidney. For example, maximal rates of oxidation were observed only if the particles were supplemented with cytochrome c and (with the exception of succinate as substrate) with DPN. The oxidation of succinate and glutamate was coupled with the synthesis of ATP. In the presence of hexokinase and fructose, the P: O ratios associated with the oxidation of these substrates were similar to those found with liver mitochondria, provided that fluoride was added to the reaction mixture. The dephosphorylation of adenosine polyphosphates by tumor particles was very rapid. The oxidative phosphorylations carried out by tumor mitochondria were abolished by low concentrations of di-nitrophenol, and it was shown conclusively that they were not artifacts due to contaminating glycolytic reactions. Further evidence that tumor mitochondria catalyzed true oxidative phosphorylations was obtained from studies on the incorporation of P32-labeled inorganic phosphate into various acid-insoluble components of these particles. Under conditions propitious for oxidative phosphorylation, extensive entry of P32 into the phospholipids and phosphoproteins of the mitochondria of the Flexner-Jobling rat carcinoma was observed. These synthetic reactions were dependent upon the addition of tricarboxylic acid cycle substrates and were extremely sensitive to dinitrophenol. It was also found that the presence of large, actively growing, transplantable tumors did not affect the efficiency of oxidative phosphorylation by rat-liver mitochondria.

Experiments with the Ehrlich ascites tumor (cf. 128) suggested strongly that intact cancer cells carry out oxidative phosphorylations and that the ATP synthesized thereby can be used for vital synthetic reactions. When thoroughly washed ascites cancer cells were incubated in air with inorganic phosphate labeled with P32, the isotope was incorporated into the phospholipids, nucleic acids, and phosphoproteins of these cells at about the same rate in the absence or presence of glucose. Since the washed cells did not form any lactic acid without the addition of glucose, it seemed most likely that the endogenous respiration was coupled with the synthesis of ATP. This was corroborated by the finding that dinitrophenol strongly inhibited these synthetic reactions in the absence of glucose. Under anaerobic conditions, the incorporation of P32 into these acid-insoluble constituents of ascites cancer cells was completely dependent upon the addition of glycolyzable sugars and was barely affected by dinitrophenol. Anaerobically in the presence of glucose, the rates of these synthetic reactions were as fast as those observed without substrate in air, which reflects the high efficiency of the glycolytic synthesis of ATP by these tumor cells.

These studies established unequivocally that the energy derived from the respiration of intact tumor cells or isolated tumor mitochondria in vitro can be used for the synthesis of ATP. But the extent to which respiratory and glycolytic pathways contribute to the total energy metabolism of tumor cells growing in the body is as problematical today as it was ten years ago.