Identification and partial characterization of a somatostatin‐14 binding protein on rat liver plasma membranes

Abstract Binding of somatostatin‐14 to rat liver plasma membranes was characterized with 125 ‐labeled[tyr 11 ] somatostatin‐14. Binding at 24° C reached a plateau at 50 min and was reversible by synthetic somatostatin‐14. Scatchard analysis revealed a single class of binding sites (affinity constant = 2.4 ± 0.2 nmol/L, binding capacity = 148 ± 0.02 fmol/mg protein). Specificity for somatostatin‐14 was demonstrated by the inhibition of 125 I‐[tyr 11 ]somatostatin‐14 binding by biologically active somatostatin analogs but not by a biologically inactive somatostatin analog or unrelated peptides. The radioiodinated binding site complex could be cross‐linked with disuccinimidyl suberate. Analysis by sodium dodecyl sulfate–polyacrylamide gel electrophoresis and gel autoradiography revealed a 70,000‐Da band. Dithiothreitol, a reducing reagent, did not alter the mobility of the band, and the band could be abolished in the presence of 10 μmol/L synthetic somatostatin‐14. Covalently crosslinked, iodinated binding protein complexes could be solubilized by the nonreducing detergents Zwittergent 3–12 and 3‐([3‐cholamidopropyl] diethylammonio)‐1‐propanesulfonic acid (CHAPS). Solubilized complex bound to wheat‐germ agglutinin–agarose columns and was eluted by N, N′, N″‐triacetylchitotriose. Binding to wheat‐germ agglutinin agarose columns was lost after pretreatment with endo‐β‐ N ‐acetylglucosaminidase F. Binding studies with liver plasma membranes, 125 I‐labeled[tyrosine 11 ]somatostatin‐14 and guanine nucleotides showed inhibition of binding in the presence of guanine nucleotides. These results indicate that the purified rat liver plasma membranes contain a specific binding protein for somatostatin‐14, the binding protein appears to be glycosylated and somatostatin‐14 binding to rat liver plasma membranes may be regulated by G proteins. (H EPATOLOGY 1992;16:433–439.)