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.