Previous studies have suggested that copper is incompletely incorporated into caeruloplasmin, the major plasma form of copper-transporting protein, in the genetic copper toxic condition, Wilson's disease. In this paper we have investigated the role of copper and caeruloplasmin in the abnormal biliary copper transport that characterizes Wilson's disease. Using SDS/PAGE and Western blotting, we have demonstrated the presence of holocaeruloplasmin in liver samples from patients with Wilson's disease (abnormal biliary copper excretion) and in control patients (normal biliary copper excretion). The presence of holocaeruloplasmin was also confirmed by measurement of caeruloplasmin oxidase activity using staining with o'Dianisidine. In contrast with the findings in liver tissue, holocaeruloplasmin was absent from bile from patients with Wilson's disease, but as expected it was present in the bile from control subjects. We have also identified and partially characterized a 189–200 kDa protein from purified human biliary canalicular membranes which binds copper and possesses caeruloplasmin-like activity when probed with a specific human anti-caeruloplasmin antibody. In conclusion, we have demonstrated that copper incorporation in caeruloplasmin is normal in patients with Wilson's disease contrary to previous reports. Secondly, we have shown that the defect in Wilson's disease appears to lie in the biliary canalicular excretion of holocaeruloplasmin resulting in its retention within the hepatocyte causing copper toxicosis. Finally we have identified and partially characterized a caeruloplasmin-binding protein from biliary canalicular membranes which may provide a link to the gene defect in Wilson's disease.

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