1. Addition of cyanide to slices of Walker carcinosarcoma 256, incubated in an oxygenated medium and utilizing endogenous substrates, causes the transition of cytochrome b from an oxidized to a reduced steady state. A further reversible reduction of 20-40% occurs upon changing the gas phase from O2 to N2. 2. Nicotinamide adenine dinucleotides and flavoproteins (but not cytochrome c + c1 or a + a3) also undergo reversible redox changes during normoxia-anoxia cycles in the presence of cyanide. 3. The extra reduction of cytochrome b and flavoproteins induced by anaerobiosis in the presence of cyanide is reversed by pyruvate. 4. Electron microscopic examination of the Walker tumour shows the presence of a considerable amount of Golgi membranes but a very low content in smooth endoplasmic reticulum. 5. Spectrophotometric analysis of the microsomal fraction of the tumour indicates the presence of cytochrome b5 and another, unidentified b-type pigment (peaks at 559, 530 and 427 nm), whereas cytochrome P-450 is lacking. 6. The cyanide-insensitive redox changes of electron carriers described above are attributed to an electron transport system involving the cytochrome b-type pigments of the microsomal fraction.
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