Solubilization and characterization of guinea‐pig pancreatic somatostatin receptors

The solubilization of somatostatin receptors from guinea‐pig pancreas by different non‐denaturing detergents was investigated after stabilization of the receptors by prior binding of 125 I‐[Tyr 11 ]somatostatin or its analogue 125 I‐[Leu 8 , DTrp 22 , Tyr 25 ]somatostatin 28, to pancreatic plasma membranes. The somatostatin‐receptor complexes were solubilized in a high yield by Zwittergent 3–14 (3‐[tetradecyldimethylammonio]‐1‐propanesulfonate), a zwitterionic detergent. Other detergents, digitonin, Triton X‐100, Chaps (3‐[cholamidopropyldimethylammonio]‐1‐propanesulfonate) and octyl β‐D‐glycopyranoside, achieved only partial solubilization. The recovery of receptor complexes was increased by glycerol. In order to characterize solubilized somatostatin‐receptor comples, membranes receptors were covalently labelled using N ‐5‐azido‐2‐nitrobenzoyloxysuccinimide as cross‐linking reagent before solubilization. Gel filtration chromatography analysis resulted in the identification of a major protein component of apparent M r = 93000 which interacted with the two radioligands. In addition, a similar component of M r = 88000 was characterized after analysis by SDS‐PAGE of membrane receptors covalently cross‐linked with 125 I‐[Leu 8 , DTrp 22 , Tyr 25 ]somatostatin 28 by different heterobifunctional reagents: N ‐5‐azido‐2‐nitrobenzoyloxysuccinimide, N ‐hydroxysuccinimidyl 4‐azidobenzoate, N ‐succinimidyl 6‐(4′‐azido‐2′‐nitrophenylamino)hexanoate. Optimal cross‐linking results were obtained with N ‐5‐azido‐2‐nitrobenzoloxysuccinimide. The solubilized somatostatin‐receptor complex was adsorbed to wheat‐germ agglutinin‐agarose column and eluted by specific sugars. We concluded that the guinea‐pig pancreatic somatostatin receptor in the membrane and in the non‐denaturing detergent solution behaves as a protein monomer of apparent M r ∼ 85000–90000. The somatostatin receptor is a glycoprotein which contains complex‐type carbohydrate chains.