Monoamine oxidase of types A and B in the saphenous vein and mesenteric artery of the dog.

Homogenates of dog saphenous vein and mesenteric artery were prepared in phosphate buffer. Monoamine oxidase (MAO) activity was determined with 3H-5-hydroxytryptamine (3H-5-HT) as a preferential substrate for MAO type A, 14C-beta-phenylethylamine as a preferential substrate for MAO type B, and 3H-tyramine as a substrate for both MAO types. Km and Vmax for the different substrates were determined, and clorgyline and (-)-deprenyl were used as specific inhibitors. The endogenous noradrenaline content was compared with the activity of MAO in both blood vessels. The results show that the enzymatic deamination of tyramine is slightly but significantly higher in the mesenteric artery than in the saphenous vein. MAO A activity was significantly higher in the mesenteric artery than in the saphenous vein, but MAO B activity was the same in both vessels. Hence, the ratio MAO A activity/MAO B activity was greater for the mesenteric artery than for the saphenous vein. This difference may be related to the density of the adrenergic innervation of the two blood vessels.