Pancreatic CCK receptors: characterization of covalently labeled subunits

125I-CCK was crosslinked with ultraviolet light to its receptor on pancreatic plasma membranes. The predominant labeled species following polyacrylamide gel electrophoresis had a molecular weight of 120,000 in the absence, and 80,000 in the presence of the reducing agent dithiothreitol. The M = 120,000 labeled band could be extracted, reduced and converted to Mr = 80,000. Moreover, peptide mapping with Staph aureus V8 protease showed a similar pattern for the 120,000 and 80,000 dalton bands. The crosslinked receptor could be solubilized with Triton X-100, absorbed to wheat germ agglutinin and eluted with N-acetylglucosamine. The results indicate, therefore, that the CCK receptor is a glycoprotein with subunits coupled by disulfide bonds.