Solubilization and partial purification of somatostatin-28 preferring receptors from hamster pancreatic beta cells.

Somatostatin-28 (SRIF-28) preferring receptors were solubilized from hamster beta cell insulinoma using the zwitterionic detergent 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate. The binding of the iodinated [Leu8-D-TRP22-Tyr25]SRIF-28 analog (referred to as 125I[LWY] SRIF-28) to the solubilized fraction was time-dependent, saturable, and reversible. Scatchard analysis of equilibrium binding data indicated that the solubilized extract contained two classes of SRIF-28-binding sites: a high affinity site (Kd = 0.3 nM and Bmax = 1 pmol/mg protein) and a low affinity site (Kd = 13 nM and Bmax = 4.7 pmol/mg protein). The binding of 125I[LWY]SRIF-28 to solubilized SRIF-28 receptors was sensitive to the GTP analog guanosine-5'-O-thiotriphosphate, suggesting that receptors are functionally linked to a G-protein. By anion-exchange chromatography of the solubilized extract followed by chromatography on wheat germ agglutinin, a 46-fold purification of SRIF-28 receptors was obtained. At this stage of purification, only high affinity sites were found (Kd = 1 nM) and the GTP effect was not maintained. A specific protein of 37 kDa was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after photoaffinity labeling. We suggest that this protein is the putative SRIF-28 receptor or a subunit thereof.