Kinetic measurements of a novel copper-dependent amine oxidase, purified from rat liver mitochondria matrix, were carried out using various substrates in a large pH (5.6-10.2) and ionic strength range (5-200 mM), in order to study the docking of substrates to the enzyme and, as a consequence, to verify the physicochemical characteristics of the active site. Relatively small changes of V (max) values (approx. 2.5-folds) over the substrates tested, suggest that the rate determining step of the catalysis is only slightly affected by amine chemical structure. In contrast, the strong change of K (M) and k (c)/K (M) values (approx. two orders of magnitude) indicates electrostatic control of the docking process, since the changes of K (M) and k (c)/K (M) values appear due to the presence of positively charged groups in the substrate molecules. These results suggest the presence in the enzyme active site of two negatively charged amino acid residues which seem to interact with positively charged groups of the substrate molecules. Analogies and differences with bovine serum amine oxidase are also described.