Inhibition by cyclopropylamine of the quinoprotein methylamine dehydrogenase is mechanism-based and causes covalent cross-linking of alpha and beta subunits

Cyclopropylamine acted as a mechanism-based inhibitor of the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans. The protein-bound quinone cofactor of this enzyme was rapidly reduced by addition of a stoichiometric amount of cyclopropylamine, but this compound did not serve as a substrate for the enzyme in the steady-state kinetic assay. Time-dependent inactivation of the enzyme by cyclopropylamine was observed only in the presence of a reoxidant. Saturation behavior was observed, and values of KI of 3.9 microM and K(inact) of 1.7 min-1 were determined. Enzyme inactivation was irreversible, as no restoration of activity was evident after gel filtration of methylamine dehydrogenase which had been incubated with cyclopropylamine in the presence of a reoxidant. The inactivated enzyme exhibited an altered absorption spectrum. Electrophoretic analysis of inactivated methylamine dehydrogenase indicated that covalent cross-linking of the alpha and beta subunits of this alpha 2 beta 2 oligomeric enzyme had occurred and that the quinone cofactor had been modified. A mechanism for this inhibition is proposed which is based upon the data presented and is consistent with the available structural information on methylamine dehydrogenase.