Summary: The modifier protein (M-protein) for methanol dehydrogenase (MDH) of the obligate methylotroph was purified almost to homogeneity (98% pure), characterized and shown to be similar to that previously described in the facultative methylotroph AM1. It was a dimer of 140000, its isoelectric point was 5·6 and it showed no spectral absorbance above 320 nm. In the dye-linked MDH assay system, the M-proteins facilitated oxidation of alcohols not normally oxidized (1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol and 3-methylbutanol) by increasing the affinity of MDH for these alcohols. The effect of M-protein on the oxidation of formaldehyde was to decrease the affinity of MDH for formaldehyde by more than 97%, thus preventing any significant oxidation of formaldehyde. The M-protein also exerted its effect on the activity of MDH in the cytochrome-linked system of The usual function of M-protein in methylotrophs is apparently to prevent formaldehyde oxidation. A dye-linked aldehyde dehydrogenase of wide specificity was partially purified and characterized; it failed to oxidize formaldehyde at a significant rate but it oxidized lactaldehyde at a rate sufficient to account for the oxidation of propanediol to lactate in whole bacteria.


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