[Purification, characterization and clinical aspects of a liver specific antigen]

Abstract

The early interest in the identification of human liver antigens relates to their potential value as serological markers of liver cell destruction and was confined mainly to those antigens which were tissue specific. Recently this interest has widened to include organ-specific antigens which could serve as possible targets of immune attack. In this respect, several liver-specific antigens have been reported. Those of current major interest are LSP (LP-I) and LP-II (Meyer zum Büschenfelde et al., 1972), LM-Ag (Meyer zum Büschenfelde et al., 1976) and F-antigen (Fravi et al., 1968). This investigation deals with the isolation, characterization and clinical aspects of a new liver specific antigen, the basic liver protein (BLP), which is different from those previously reported. Rat BLP has been purified from liver by a combination of ammonium sulfate fractionation, DEAE-cellulose column chromatography and affinity chromatography using the IgG fraction of rabbit anti-rat BLP serum. Human BLP has been also purified using essentially the same method. The purified rat and human BLP gave molecular weights of approximately 105,000 daltons on Sephadex G-200 gel filtration and 35,000 upon SDS-polyacrylamide gel electrophoresis. These results indicate that BLP exists as a trimer. Ammino acid analyses support this possibility. The isoelectric point of rat BLP was pH 8.5 and that of the human protein pH 8.1. The molecular weights, amino acid contents, isoelectric points and other biochemical characteristics of previously reported liver specific antigens differ significantly from those of rat and human BLP. Rabbit anti-rat BLP serum gave positive reactions with the liver extracts of other animals, including mouse, human, sheep, horse, cow and rabbit, but did not react with the extracts of other tissues. This indicates that rat and human BLP are organ specific proteins which are closely related to a similar protein in other species. BLP appeared within 6 hrs in the sera of rats following treatment CCl4, increased to a maximum at 36 hrs and disappeared within 72 hrs. These experiments suggest that serum BLP levels may serve as a measure of liver damage. The IgG fraction of rabbit anti-rat BLP serum inhibited the growth of fetal rat hepatocytes. Furthermore, rabbit antibody to rat BLP reacted with rabbit BLP. These latter results suggest that BLP may be involved in auto-immune liver disease. Thus, studies of BLP may serve to provide information on the extent of damage and the nature of auto-immune mechanisms in the liver.