Summary: Pure methanol dehydrogenase of AM 1 oxidizes propanediol in a transitory fashion when measured in a dye-linked assay with phenazine ethosulphate (PES) as electron acceptor. It was shown that the transitory nature of this oxidation is not because the product (lactaldehyde) is an inhibitor but because the dehydrogenase is inactivated by PES. Substrates having low affinities for the enzyme, such as propanediol, are unable to protect against this inactivation. A ‘stimulatory factor’, previously shown to facilitate the continuous oxidation by methanol dehydrogenase of propanediol to lactate, was shown to consist of two proteins, a modifier protein (M-protein) and an aldehyde dehydrogenase. The M-protein increased the affinity of methanol dehydrogenase for propanediol; this enabled the substrate to protect the enzyme against inactivation by PES, and it facilitated the continuous oxidation of low concentrations of propanediol to lactaldehyde. The M-protein was purified almost to homogeneity (more than 95% pure) and shown to be an acidic, dimeric protein having subunits of molecular weight 64000. The second protein component of the ‘stimulatory factor’ was a dye-linked aldehyde dehydrogenase which oxidized lactaldehyde to lactate but which had a greater affinity for longer-chain aliphatic aldehydes and the following characteristics: pH optimum, 7.0; isoelectric point, 4.0; molecular weight, 115000; and for lactaldehyde, 16 mM.


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