Bilirubin binding activity of cytokeratin 18 isolated from the porcine liver

A porcine liver 40 kDa protein designated SBP40 isolated by affinity chroma tography with agarose-linked spermine was identified as a porcine cytokerat in 18 on the basis of partial amino acid sequences of peptides derived by l ysylendopeptidase digestion and by its reactivity with two commercially ava ilable preparations of monoclonal antibody. Immunohistochemistry revealed t hat SBP40 is localized at the hepatocyte membranes, preferentially in the b ile canalicular area in accordance with the previously reported localizatio n of cytokertain 18 in the murine liver. Affinity chromatography with agaro se-linked bilirubin, a solubilization experiment of bilirubin from bilirubi n-Sephadex G-10 complex, and gel-filtration of a mixture with bilirubin dem onstrated that SBP40 or porcine cytokeratin 18 has binding affinity for bil irubin. These results suggest that cytokeratin 18 may play a role as a memb rane reservoir in the event of transport and secretion of bile pigments in the liver.