Strikingly high respiratory quotients: a further characteristic of the tumor pathophysiome

Conspicuously high respiratory quotients (RQs) are found in solid tumors in vivo. RQs in the range between 1.29 and 1.95 neither reflect the degree of substrate oxidation by tumor cells nor indicate the types of fuels involved in metabolic processes. Instead, such tumor RQs most probably are caused by (a) channeling of glycolytic end-products into lipogenesis, and by (b) CO2 release from the tumor following extracellular buffering of H(+)-ions by bicarbonate. H(+)-ions exported from the intracellular space into the interstitial compartment titrate extracellular bicarbonate to CO2 and H2O with the aid of the ectoenzyme carbonic anhydrase IX, which is activated at low pH. Strikingly high (RQs) may thus be a further characteristic of the tumor microenvironment and of the tumor (patho-)physiome, the latter quantitatively describing the pathophysiologic characteristics of tumor cells and solid tumors.