Mechanistic studies of para-substituted N,N′-dibenzyl-1,4- diaminobutanes as substrates for a mammalian polyamine oxidase

The kinetics of oxidation of a series of para-substituted N,N′-dibenzyl-1,4-diaminobutanes by the flavoprotein polyamine oxidase from mouse have been determined to gain insight into the mechanism of amine oxidation by this member of the monoamine oxidase structural family. The k _cat/K_m values are maximal at pH 9, consistent with the singly charged substrate being the active form. The rate constant for flavin reduction, k_red, by N,N′-dibenzyl-1,4-diaminobutane decreases about 5-fold below a pK_a of ∼8; this is attributed to the need for a neutral nitrogen at the site of oxidation. The k_red and k _cat values are comparable for each of the N,N′-dibenzyl-1,4- diaminobutanes, consistent with rate-limiting reduction. The deuterium kinetic isotope effects on k_red and k_cat are identical for each of the N,N′-dibenzyl-1,4-diaminobutanes, consistent with rate-limiting cleavage of the substrate CH bond. The k_red values for seven different para-substituted N,N′-dibenzyl-1,4-diaminobutanes correlate with a combination of the van der Waals volume and σ value of the substrates, with ρ values of -0.59 at pH 8.6 and -0.09 at pH 6.6. These results are consistent with direct transfer of a hydride from the neutral CN bond of the substrate to the flavin as the mechanism of polyamine oxidase.