Purification and characterization of two forms of methanol dehydrogenases from a marine methylotroph

Two methanol dehydrogenases (MDHs), MDH1 and MDH2, were purified from a marine methylotroph, Methylophaga sp. strain 1. Both enzymes had very similar properties, including the same native molecular weight, sizes of subunits and substrate specificity. The N-terminal amino acid sequence of the alpha-subunit of MDH2 differed from that of MDH1 by having a histidine residue at a highly conserved glutamate position, but both sequences showed approximately 50% homology to the alpha-subunits of other MDHs. MDH1 had higher specific activity than MDH2 with respect to methanol and ethanol as a substrate. The two enzymes did not appear to be isoforms but that either MDH1 or MDH2 could be a mutant arising from spontaneous mutation.