Abstract
Photoaffinity labeling experiments were performed with membranes from rat liver containing V1 vasopressin receptors. Photoreactive analogues of [1-beta-mercaptopropionic acid]vasopressin [( Mpa1], vasopressin, or deamino-vasopressin) retaining a high binding affinity (apparent dissociation constants: 5 X 10(-9) M-3 X 10(-8) M) and agonistic properties were used. The tritium-labeled analogue [Mpa1,Lys(N epsilon-4-azidobenzoyl)8]vasopressin preferentially and specifically labels a 30-kDa polypeptide and with lower efficiency a 38-kDa polypeptide. The analogue [Mpa1,Dab4(N gamma-(N-4-azido-2-nitrophenyl-beta-Ala4]arginine-vasopressin specifically labels the 38-kDa polypeptide. The labeling of these two membrane proteins is completely suppressed by an excess of arginine-vasopressin; bradykinin or angiotensin II do not inhibit the incorporation of the reactive vasopressin analogues into these proteins. The results suggest that the rat hepatic V1 receptor exists in the plasma membrane in an oligomeric form composed of two subunits with a molecular mass of 30 and 38 kDa.
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