Time-dependent activation of the semicarbazide-sensitive amine oxidase (SSAO) from ox lung microsomes

The activity of ox lung microsomal semicarbazide-sensitive amine oxidase (EC 1.4.3.6; SSAO) towards benzylamine increased 20-fold during incubation at 37 °C. After an initial lag-period, activation was first-order with time and complete after approx. 20 h. No significant changes in activity towards methylamine, histamine or 2-phenylethylamine were observed, although mixed-substrate experiments were consistent with the same enzyme being involved in the oxidation of all these substrates, both before and after time-dependent activation. The enzyme-tryptophan fluorescence increased on incubation at 37 °C in parallel with the increase in activity towards benzylamine. Treatment of the activated-enzyme preparation with 6 M guanidinium chloride followed by dialysis, caused both the activity towards benzylamine and the fluorescence to fall to that occurring before activation. However, incubation of this preparation at 37 °C resulted in increases in fluorescence and activity similar to those seen with the unactivated enzyme. Benzylamine oxidation was inhibited, uncompetitively with respect to oxygen, by high substrate concentrations but no such inhibition was observed with the other amines. Activation resulted in an increase in V(max) for benzylamine oxidation, with no significant alterations in the K(m) or the K(si) for high-substrate inhibition. Kinetic studies were consistent with sequential mechanisms being followed for the oxidation of both benzylamine and methylamine but the dependence on oxygen concentration was complex. These results might indicate that benzylamine follows a different reaction pathway from the other substrates, with substrate-specific activation involving a reaction step that is rate-limiting for benzylamine oxidation but not for the others.