The relationship between liver-specific lipoprotein and the hepatocyte plasma membrane

Abstract

Liver-specific lipoprotein (LSP) has been the subject of intense investigation as a candidate target antigen in chronic active hepatitis. Fundamental to the interest in LSP has been the belief that it is an antigen complex of hepatocyte plasma membrane origin. In this study the physical, biochemical and antigenic relationships between LSP and isolated hepatocyte plasma membrane (HPM) were investigated. Electron microscopic examination of LSP showed it to be devoid of plasmalemma sheets that were abundant in HPM. The plasma membrane marker enzyme 5'-nucleotidase was enriched 11-fold in HPM relative to liver homogenate, whereas the enzyme activity in LSP was 17% of that found in liver homogenate and only 1.5% of that found in HPM. The antigenic relationship between LSP and HPM was assessed using sera from rabbits immunized with either mouse LSP or mouse HPM. By filtration ELISA, antibody to LSP reacted poorly with entrapped HPM, relative to antibody to mouse HPM. Antisera to LSP and HPM were both effectively absorbed by the immunizing antigen, however antibody to LSP was not absorbed with HPM, and minimal cross-absorption of HPM antibody with LSP was found. By immunoblot of SDS-PAGE separated LSP and HPM, it was shown that antigenic cross-reactivity between LSP and HPM at the polypeptide level was rare. By immunofluorescence, antibody to LSP failed to react with the surface of viable mouse hepatocytes, whereas antibody to HPM showed linear fluorescence. The data show that the two preparations, LSP and HPM, are dissimilar antigen complexes. HPM may be a more appropriate preparation for the study of autoimmune liver disease than LSP.