Fractionation of the liver membrane lipoprotein (LSP) and characterisation of its antigenic determinants by autoantibodies and a heterologous anti-serum.

Sera from 50 patients with chronic active liver disease were investigated by radioimmunoprecipitation test for autoantibodies against human and rabbit liver membrane lipoprotein (LSP), and a human kidney equivalent protein. Twenty-one of 50 chronic active liver disease sera were positive for autoantibodies against human liver membrane lipoprotein. Eleven of 50 sera reacted with rabbit liver membrane lipoprotein and four of 50 sera with human kidney equivalent protein as well. I order to identify the antigenic determinants of these autoantibodies 125I-labelled human and rabbit liver membrane lipoprotein and human kidney equivalent protein were fractionated by CsCl density gradient centrifugation. Distribution of labelled antigen fractions in density gradients revealed two peaks with maximum radioactivity at 1.34 and 1.11 g/ml. Density gradient fractions were assayed for antigen activity by radioimmunoprecipitation test using autoantibody positive sera of patients with chronic active liver disease. Anti-human and anti-rabbit liver membrane lipoprotein and anti-human kidney equivalent protein positive human sera all localised their corresponding determinants in the low density fractions (1.08-1.10 g/ml). An antiserum against human liver membrane lipoprotein, raised in rabbits after short-term immunisation, recognised the low density 125I-human liver membrane lipoprotein subfraction. This serum had previously been absorbed with human kidney homogenate, plasma, and blood cells. Thus, organ-specific and non-organ specific determinants were both localised in the low density liver membrane lipoprotein subfraction. They could not be separated by the described fractionation procedure. It is to be supposed that organ-specific determinants of a low density membrane protein of hepatocytes are targets circulating autoantibodies in chronic active liver disease.