Bilirubin Binding Activity of Cytokeratin 18 Isolated from the Porcine Liver

Abstract

A porcine liver 40 kDa protein designated SBP40 isolated by affinity chromatography with agarose-linked spermine was identified as a porcine cytokeratin 18 on the basis of partial amino acid sequences of peptides derived by lysylendopeptidase digestion and by its reactivity with two commercially available preparations of monoclonal antibody. Immunohistochemistry revealed that SBP40 is localized at the hepatocyte membranes, preferentially in the bile canalicular area in accordance with the previously reported localization of cytokertain 18 in the murine liver. Affinity chromatography with agarose-linked bilirubin, a solubilization experiment of bilirubin from bilirubin-Sephadex G-10 complex, and gel-filtration of a mixture with bilirubin demonstrated that SBP40 or porcine cytokeratin 18 has binding affinity for bilirubin. These results suggest that cytokeratin 18 may play a role as a membrane reservoir in the event of transport and secretion of bile pigments in the liver.