Tetranitromethane (TNM) was employed as an electrophilic reagent to probe for the lysyl oxidase-catalyzed processing of n-butylamine to an intermediate carbanion during the oxidation of this amine to n-butyraldehyde according to a prior description of the use of TNM to trap enzyme-generated carbanion intermediates (Christen, P. and Riordan, J. F. (1968) Biochemistry 7, 1531-1538). The addition of n-butylamine to assay mixtures containing lysyl oxidase and TNM markedly increased the background rate of nitroform release. The Km for n-butylamine was essentially the same whether determined from the rate of lysyl oxidase-catalyzed nitroform release or from the rate of n-butyraldehyde production in the absence of TNM, the latter assessed by measurements of the rate of H2O2 formation. The initial rate of substrate- and enzyme-dependent nitroform production was linearly related to functional active site content. These data are consistent with the enzyme-dependent abstraction of an alpha-proton from the substrate to form an intermediate enzyme-bound carbanion. Kinetic analyses of the oxidation of n-butylamine and 1,1-dideutero-n-butylamine by lysyl oxidase revealed kinetic isotope effects on Vmax and Vmax/Km parameters, consistent with a rate-contributing alpha-proton abstraction step. These and other available data are incorporated into a proposal for the mechanism of action of this enzyme.