Hepatic processing of cholecystokinin peptides. II. Cellular metabolism, transport, and biliary excretion

We have shown that radiolabeled cholecystokinin octapeptide (CCK-8), CCK-8-desulfate, and CCK-4 are extracted by the liver in a structurally specific manner. Thus, we studied the fate of the extracted radiolabeled peptides by quantitating biliary excretion and determining the nature of the metabolites in bile. There was rapid biliary excretion of labeled CCK-8, CCK-8-desulfate, and CCK-4 by the isolated, perfused rat liver; greater than 75% of the extracted dose and greater than 20% of the injected dose appeared in bile within 20 min after a single pass across the liver. By means of high-performance liquid chromatography and immunoprecipitation, we showed that CCK-8-desulfate and CCK-4 appeared in bile in completely metabolized forms. In contrast, for CCK-8, ~20% of the major forms of the label in bile was intact labeled octapeptide. To gain insight into the subcellular sites of metabolism and transhepatic transport of CCK-8, we also determined the effects of taurocholate, lysosomotropic agents, and microtubule binding agents on biliary excretion. Taurocholate had no effect on the percentage of the extracted label excreted into bile. Neither lysosomotropic agent, leupeptin, nor chloroquine affected the percentage of extracted label or the nature of the metabolites appearing in bile. Two microtubule binding agents, vinblastine and colchicine, also did not affect the percentage of the extracted label appearing in bile. These results suggest that, after efficient first-pass hepatic extraction, cholecystokinin undergoes extensive metabolism, possibly at a subcellular site other than lysosomes, and is rapidly and efficiently transported across the hepatocyte into bile, possibly by a nonvesicular transport process.