Binding of somatostatin-14 to rat liver plasma membranes was characterized with 125-labeled[tyr11] 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 125I-[tyr11]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, 125I-labeled[tyrosine11]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. (HEPATOLOGY 1992;16:433–439.)