1. Mitochondria-enriched fractions of the soil amoeba Acanthamoeba castellanii contained four haemoproteins that in their reduced forms reacted with CO to give photodissociable CO complexes; these were cytochromes a 3, a 614, b- and c-type cytochromes. 2. Non-photodissociable oxygen-containing compounds were formed at temperatures between -130 and -150 degrees C after photodissociation of CO in the presence of 200 microM-O2, 3. Electron transport, indicated by the oxidation of cytochromes a + a3 and cytochrome c, did not occur until the temperature was raised to -80 degrees C.
