Methylamine oxidase (EC 1.4.3.6) from Arthrobacter P1 was inactivated by NaCNBH3 in the presence of [14C]benzylamine, leading to the incorporation of 1 mol of radiolabeled substrate/mol of enzyme subunit at complete inactivation. By contrast, no labeling of enzyme was observed using [3H]NaCNBH3 as reductant. These results are analogous to those previously reported for the eukaryotic enzyme, bovine serum plasma anaine oxidase [(1987) J. Biol. Chem. 262, 962-965]. The observed pattern of labeling is consistent with the presence of dicarbonyl cofactor at the active site of methylamine oxidase. Further, these studies suggest that our reductive trapping technique, in which the pattern of radiolabeling of an enzyme is compared using C-14 substrate vs tritiated reductant, may serve as a general assay for covalently bound dicarbonyl structures.
