BINDING-PROTEINS FOR CYCLOSOMATOSTATINS AND BILE-ACIDS IN BASOLATERALPLASMA-MEMBRANES OF RAT-LIVER

The bile acids cholate and taurocholate on the one hand and the cyclop eptide c(Phe-Thr-Lys-Trp-Phe-D-Pro) (008), an analog of somatostatin w ith retro sequence, on the other hand, display mutually competitive tr ansport inhibition into isolated rat hepatocytes. This indicates a com mon transport system for bile acids and cyclosomatostatins in sinusoid al rat liver plasma membranes. In order to identify and isolate common binding and/or transport proteins for bile acids and the cyclopeptide s by affinity chromatography, the bile acid derivative 4'-amino-7-benz amidotaurocholate (ABATC) and the cyclosomatostatin-analog 008 were at tached to a gel matrix. Two methods were used to prepare integral memb rane proteins: (1) alkaline EDTA extraction and (2) Triton X-114 phase separation. Octyl glycoside solubilized, alkaline EDTA-extracted inte gral basolateral membrane proteins with apparent molecular masses of 5 2 and 48 kDa bound specifically to the ABATC affinity matrix. Two-phas e Triton X-114 separated integral membrane proteins of the same molecu lar masses bound specifically to the cyclosomatostatin ligand. The 48 kDa ABATC and 008 binding protein was shown to be present in the basol ateral plasma membrane fraction and in the microsomal fraction. The is olated 52 kDa ABATC binding protein was localized only in basolateral plasma membranes and could not be found in isolated microsomes.