Evidence for different forms of monoamine oxidase in perfused rabbit lung

In lungs perfused with ¹⁴C phenylethylamine (¹⁴C-PEA) alone 60 to 80% or total radioactivity in the venous effluent was in the form of deaminated products. The treatment with 0.01 mM pargyline reduced PEA deamination by approximately 60%; increasing pargyline concentration to 1.0 mM failed to increase the percent inhibition of PEA deamination. In the presence of the plasma amine oxidase inhibitor, semicarbazide, a mean value of 28% inhibition of PEA deamination was achieved. Treatment of lungs simultaneously with pargyline at either 0.01 or 1.0 mM concentration and semicarbazide (1.0 mM) completely prevented deamination of PEA. These experiments clearly demonstrate the existence of both the mitochondrial type B and the plasma forms of amine oxidase in the intact perfused lung. The production of 5 hydroxyindole acetic acid in the presence of harmaline was progressively reduced to about 30% of its original value while the acid metabolite of phenylethylamine was produced at its normal rate despite the presence of this inhibitor. It must be emphasized that during harmaline perfusion the uptake of ¹⁴C-PEA or ¹⁴C-5 hydroxy tryptamine into the lung was unaltered. Accordingly, it must be concluded that harmaline inhibited the type A form of monoamine oxidase (MAO) in lung. The authors believe these data represent definitive evidence for the existence of 3 forms of amine oxidase in an intact perfused organ. Although the location of the different forms of enzyme cannot be determined from this study, previous work has demonstrated that following inhibition of MAO by pargyline, both 5-HT and norepinephrine fluorescence is found predominantly in the capillary endothelium of rabbit lung. Therefore, it is possible that all 3 forms of the amine oxidase are associated with this cell type.