ISOLATION AND CHARACTERIZATION OF THE RAT LIVER AVP RECEPTOR USING [¹²⁵I][d(CH₂)_5′SARCOSINE⁷]AVP¹

1. A vasopressin (AVP) binding protein was purified from rat liver membranes by an improved method using [¹²⁵I][d(CH₂)_5′Sarcosine⁷]AVP, a selective Vi AVP radioligand and a combination of CHAPS solubilization, gel filtration, lectin affinity and FPLC ion exchange chromatography. 2. The purified protein exhibited a maximum binding activity of 2480 pmol/ mg protein with a K_D of 4.5 nmol/ L, which corresponds to a purification of approximately 26 700‐fold. The molecular weight of this protein was 70 000 Da. 3. The binding of [¹²⁵I][d(CH₂)_5′Sarcosine⁷]AVP to the solubilized membranes was dependent on the protein concentration, and was inhibited by the unlabelled peptides [d(CH₂)_5′Sarcosine⁷]AVP, AVP, and to a lesser degree by peptides with high V₂ receptor affinity, such as 1‐desamino‐D‐AVP and [d(CH₂)_5′D‐Ileu²‐Ileu⁴]AVP. 4. In addition, an AVP anti‐idiotypic monoclonal antibody bound to both the partially purified and purified lectin affinity AVP binding protein in a concentration‐dependent manner. These results indicate that the purified protein displays similar characteristics to the liver membrane‐bound AVP V₁ receptor.