= Methanol dehydrogenase
A \b[methanol dehydrogenase] is an <enzyme> that catalyzes <oxidation> of <methanol> to <formaldehyde>, transferring reducing equivalents to an acceptor. The name covers different enzyme activities and does not identify a single universal <cofactor (biochemistry)> system. The NAD-dependent activity has the net reaction
$$
\mathrm{CH_3OH+NAD^{+}\rightleftharpoons HCHO+NADH+H^{+}}.
$$
Here <nicotinamide adenine dinucleotide> accepts reducing equivalents. A different classified activity transfers them to <cytochrome c> and uses <pyrroloquinoline quinone> with <calcium> at its <active site>.<Enzyme Nomenclature by the IUBMB>[EC1.1.1.244, reaction; EC1.1.2.7, reaction and Comments]{ref}
An atom-and-charge balance makes the oxidation explicit:
$$
\mathrm{CH_3OH\longrightarrow HCHO+2H^{+}+2e^{-}}.
$$
Removing two electrons from the substrate half-reaction does not specify their final destination. That destination belongs to the enzyme system and its available acceptors. Consequently, an assay designed for an NAD-linked activity cannot simply be assumed to report the cytochrome-linked activity.
The 1964 study by <C. Anthony> and <L. J. Zatman> illustrates the experimental issue. Their coupled assay used <phenazine methosulfate>; it was independent of nicotinamide nucleotide coenzymes. To test product identity, the investigators separated material by steam distillation before the formaldehyde color reaction because the coupled-assay reagent interfered with direct measurement. They explicitly left open whether free formaldehyde was produced initially or released from a bound form during analysis.<The microbial oxidation of methanol. 2. The methanol-oxidizing enzyme of Pseudomonas sp. M 27 by C. Anthony and L. J. Zatman>[p.614, introduction; p.620, Product of enzymic methanol oxidation and Table2]{ref}
Thus a coupled signal and an independent product measurement answer complementary questions: whether electron transfer occurs under the assay conditions, and what chemical product the analytical treatment detects.
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