Involvement of superoxide in the catalytic cycle of diamine oxidase

The reaction of pig kidney diamine oxidase (amine:oxygen oxidoreductase (deaminating) (pyridoxal-containing), EC 1.4.3.6) could be significantly inhibited by superoxide dismutase active copper chelates but not by native 2Cu,2Zn-superoxide dimutase (cuprein). The ligands alone as well as Cd2+, a heavy metal of similar toxicity to Cu2+, showed no inhibition whatsoever. This indicates that .O-2 participates in the catalytic cycle and is produced at a site scarcely accessible to such a large molecule as cuprein. A mechanism for the second, aerobic step of the diamine oxidase reaction is suggested.