Mechanism of action of putrescine oxidase. Binding characteristics of the active site of putrescine oxidase from Micrococcus rubens.

Putrescine oxidase (EC 1.4.3.4), putrescine: oxygen oxidoreductase (deaminating) (flavin containing), has been found to form complexes with a variety of amines. With few exceptions these compounds competitively inhibit putrescine oxidation and also perturb the visible absorption spectrum of the enzyme (i.e., the spectrum due to FAD). Inhibition constants are reported for a number of amines; the presence of a cationic amino group in the inhibitors appears to be the structural feature essential for competitive inhibition. Inhibition constants for amino acids are larger than those for the analogous simple amines and the inhibition constants for alkyl mono- and diamines in a homologous series are inversely related to the length of the hydrocarbon chain. Amines containing unsaturated and aromatic substituents yield relatively low inhibition constants. The spectral changes observed upon complex formation are interpreted as indicating a less polar environment for FAD in the enzyme-inhibitor complex than in the uncomplexed enzyme. On the basis of the enzyme's substrate specificity and comparisons among inhibitor structures and the corresponding inhibition constants, a schematic model of the enzyme's active site is proposed.