It has been suggested that active forms of quercetin ( o-semiquinones) are able to oxidize NADH in mammalian cells. The purpose of this study was to investigate this proposition by measuring the beta-hydroxybutyrate to acetoacetate ratio as an indicator of the mitochondrial NADH/NAD (+) redox ratio in the isolated perfused rat liver. The NADH to NAD (+) ratio was reduced by quercetin; half-maximal reduction occurred at a concentration of 32.6 microM. Additionally, quercetin (25 to 300 microM) stimulated the Krebs cycle ( (14)CO (2) production) and inhibited oxygen uptake (50 to 300 microM). Low quercetin concentrations (25 microM) stimulated oxygen uptake. The results of the present work confirm the hypothesis that quercetin is able to participate in the oxidation of NADH in mammalian cells, shifting the cellular conditions to a more oxidized state (prooxidant activity). Stimulation of the Krebs cycle was probably caused by the increased NAD (+) availability whereas the decreased NADH availability and the inhibition of mitochondrial energy transduction could be the main causes for oxygen uptake inhibition.

Studies on fatty acid and amino acid metabolism in the liver of Walker-256 tumour-bearing rats have revealed several changes. Comparisons, however, have been based on experiments performed with non-physiological, frequently unrealistic, substrate concentrations. The aim of the present work was to examine the influence of physiological substrate concentrations on gluconeogenesis, ketogenesis and related parameters. Isolated livers were perfused and substrates were infused at concentrations that were reported to occur in healthy and tumour-bearing rats. Ketogenesis and the mitochondrial NADH/NAD+ ratio were smaller in the tumour-bearing condition at low (0.2 mM) and high (0.8 mM) oleate concentrations. In the absence of oleate, gluconeogenesis from alanine (0.7 mM) and gluconeogenesis plus the associated changes in oxygen uptake due to lactate/pyruvate (2/0.2 and 6/0.3 mM) were smaller in livers of tumour-bearing rats. However, the response of gluconeogenesis from lactate/pyruvate in livers of tumour-bearing rats to 0.8 mM oleate was more pronounced so that a trend towards normalization was apparent at high substrate and oleate concentrations. Gluconeogenesis from 0.7 mM alanine was not significantly changed by oleate in the tumour-bearing state; in the control condition, stimulation occurred at 0.2 mM oleate and inhibition at 0.8 mM oleate. This diminution almost equalized the hepatic alanine-dependent gluconeogenesis of both control and tumour-bearing rats. Ureogenesis was smaller in the tumour-bearing state and was not affected by oleate. It was concluded that the high concentrations of fatty acids and lactate/pyruvate, which predominate in rats bearing the Walker-256 tumour, could be effective in normalizing the gluconeogenic response of livers from tumour-bearing rats.