Synthesis and Biological Activities of a Photoaffinity Probe for Vasotocin Receptors*

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Abstract
This study reports the synthesis and biological activities of 1-desamino,7-lysine-(4-azidobenzoyl),8-arginine vasotocin (d7-N3-AVT). This compound was found to be biologically active in the rat antidiuretic assay (20 U/mg), to behave as an antagonist of vasopressin in the rat pressor assay (pA2 = 6.6), and to yield a half-maximal hydroosmotic response in the isolated toad urinary bladder at a bath concentration of 2.4 .times. 10-8 M. When toad bladders were exposed to d7-N3-AVT in the presence of long wavelength UV light, the hydroosmotic response persisted in spite of prolonged and repeated periods of washout. By contrast, the hydroosmotic response in control bladders after stimulation with d7-N3-AVT in the absence of UV irradiation was fully reversed within 15 min of washout. A membrane preparation derived from bladders that had been photolabeled with d7-N3-AVT and washed for 1 h specifically bound 325 fmol [3H]vasopressin/mg protein. Matched bladders exposed to the analog in the absence of UV irradiation and washed for 1 h specifically bound 591 fmol [3H]vasopressin per mg of protein. These studies indicate that d7-N3-AVT binds covalently to hydroosmotic receptors of toad urinary bladder and forms a complex that is functional in triggering an increase in the permeability to water of the epithelium. This analog may prove useful in the isolation and purification of vasotocin receptors in lower vertebrates.

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