1. Mitochondria isolated from the Walker carcinosarcoma 256 contain cytochromes a, a3, b and c plus c1 in an amount corresponding to 10-20% of that present in rat-liver mitochondria. However, the relative proportions of the cytochromes are similar to those of rat liver. Flavoproteins were not detected in the tumour mitochondria. Electron microscopic examination shows that the tumour mitochondria are fewer in number and are somewhat altered morphologically. 2. Rotenone, antimycin A and cyanide inhibited the respiration of tumour slices in the presence or absence of glucose by a maximum of 85-90%. 3. Oligomycin inhibited the endogenous respiration of tumour slices by a maximum of 48.5%, half-maximal inhibition being given at a concentration of about 0.1 μg/ml. The inhibition was only partially relieved by 2,4-dinitrophenol. 4. The activity of all enzymes (except mitochondrial malate dehydrogenase) concerned in the α-glycerophosphate and malate-aspartate redox "shuttles" was greatly reduced in the tumour. In reconstitution experiments on the malate-aspartate cycle the Walker tumour mitochondria were unable to carry out an active oxidation of added NADH. These "shuttles" cannot be very active in this tumour. 5. The present results show that, compared to normal liver, some of the factors involved in the regulation of the NAD(P)+ redox potential are modified in the Walker carcinosarcoma 256. However, comparison with results obtained from other kinds of tumour cells suggests that these modifications may not be the factor responsible for changes in the oxidative metabolism of carbohydrates in this tumour.
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