The distribution of methane mono-oxygenase (MMO) activities between particulate and soluble fractions of cell-free extracts of OB3b was dependent upon growth conditions. Particulate activity was associated with the presence of intracytoplasmic membranes observed only under oxygen-limiting conditions in shake flask cultures. Particulate and soluble activities showed substantially different sensitivities to a range of potential inhibitors. The particulate enzyme was inhibited by metal-chelating agents, thiol reagents and amytal, whereas the soluble MMO was not inhibited by these compounds; both activities were sensitive to KCN, ethyne and 8-hydroxyquinoline. NAD(P)H was the only suitable electron donor. The activities were unstable at O C but the soluble enzyme could be partially stabilized by several compounds. The particulate and soluble MMO activities are compared with previously reported particulate and soluble MMO enzymes from this species and other methane oxidizers.


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