The ratio between heat flux and oxygen flux, the calorimetric ratio, is an enthalpy budget device used to identify anaerobic pathways in the presence of respiration. Ratios that are more exothermic (i.e. more negative) than the average for catabolic substrates (-450kJ mol O2 ± 5%; Thornton's rule), are usual for cells established in culture, including suspension-adapted LS-L929 mouse fibroblasts. A common reason for this is a high level of glycolysis, to produce lactate, simultaneously with aerobic pathways. To test the idea that the calorimetric-respirometric (CR) ratio is a revealing cytotoxic endpoint, LS cells grown in serum-rich medium were insulted with known metabolic poisons. Malonate, 2,4-dinitrophenol (2,4-DNP) and a mixture of antimycin A and rotenone increased the CR ratio to degrees largely explained by greater lactate flux, the CR700 values being 22μM malonate, 56μM 2,4-DNP and, for the mixture, 2μM antimycin A and 5μM rotenone. Higher concentrations of 2,4-DNP gave an “exothermic gap” for which there was no explained pathway. Iodoacetate decreased the CR ratio while inhibiting glycolysis, a result which can be explained by the hypothesis that substrates available in the serum were degraded by mitochondrial pathways and thereby substituted for the normal input from the Krebs cycle, which had been arrested by pyruvate starvation. In a balanced salt solution containing only 5.5mM glucose, the metabolic rate slowed and the CR ratio was more exothermic (CR700 = 6μM), giving a “gap” for which there was no explanation. Ten MEIC chemicals gave CR700 endpoints which closely corresponded to the order of toxicity for a battery of tests using animal cells. The CR method thus provides a good basis for investigating the mechanisms by which chemicals have toxic effects on cells.
MECHANISM OF FIXATION OF CARBON DIOXIDE IN THE KREBS CYCLE
