LOCATION OF THE ACTIVE-SITE OF RAT VASCULAR SEMICARBAZIDE-SENSITIVE AMINE OXIDASE

Semicarbazide-sensitive amine oxidase (SSAO) activity in rat vascular smooth muscle cells is associated extensively with the plasmalemma. To determine which side of the plasmalemma the active sites of these enz ymes face, the non-permeating agent, diazotised sulphanilic acid (DSA; 4.4 mM) was perfused through the isolated mesenteric arterial bed of the rat, in an attempt to inactivate only those active sites facing ex tracellularly. DSA perfusion abolished the presser responses to noradr enaline via inactivation of extracellular alpha(1) receptors but had n o effect on cytosolic lactate dehydrogenase activity. SSAO activity, e stimated by perfusing [C-14] benzylamine, was reduced following DSA pe rfusion to 55.9 +/- 4.9% of that in control beds and to 52.4 +/- 6.0% in homogenates of these vessels. These results suggest that almost hal f of SSAO active sites in rat mesenteric arteries face outwards.