SUMMARY: Both the soluble cytochromes of the obligate methylotroph were rapidly autoreducible at high pH. The intramolecular autoreduction mechanism was also involved in the reduction of the cytochrome by methanol dehydrogenase which occurred in the absence of methanol. Pure soluble methanol dehydrogenase was shown to be able to catalyse the methanol-dependent reduction of pure cytochrome from and from the facultative methylotroph Pseudomonas AM1 by coupling oxidation of the bacterial cytochrome to the reduction of a large excess of mammalian cytochrome . Only one of the two cytochromes (cytochrome of each organism) could react with methanol dehydrogenase to give methanol:cytochrome oxidoreductase activity. This activity, using proteins from was independent of pH between pH 7.0 and 9.0 and ammonia was not required. By contrast, the pH optimum for the system from Pseudomonas AM1 was 9.0 and activity was stimulated about fourfold by NHCl. The product of methanol oxidation was formaldehyde, which was also a substrate for the oxidoreductase system. During formaldehyde oxidation two molecules of cytochrome were reduced for every molecule of formaldehyde oxidized. In a survey of methanol dehydrogenases and cytochromes from Pseudomonas AM1, and the facultative autotroph it was shown that, of the two soluble cytochromes found in each methylotroph only one was able to react with methanol dehydrogenase. The cytochrome from and the cytochrome (2) of were specific, only reacting with methanol dehydrogenase from the same organism, whereas the cytochrome of AM1 reacted with all three methanol dehydrogenases tested.


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