A serum proteinase converts proapolipoprotein AI secreted by rat hepatocytes to the mature apolipoprotein.

Abstract

Apolipoprotein AI integrated into the high-density lipoprotein particle in serum was synthesized in the rat hepatocyte in the presence of radiolabelled amino acids and isolated from the cells in primary culture (suspension) as its proform, with the N-terminus extended by a hexapeptide segment. The primary secretion product is this proform which is only further proteolytically processed in the presence of the serum fraction with density higher than 1.21 g/ml. The secretion product and the proteolytically converted product were characterized by Edman degradation of their respective amino-acid sequences after radiolabelling with [3H]valine and [3H]phenylalanine, the positions of which are well established in the preproform and in the N-terminus of mature rat apolipoprotein AI. The products from the lysed cells and their culture medium were purified by immunoprecipitation, sodium dodecyl sulfate gradient gel electrophoresis and subsequent electroelution of the apo AI band. The proform sediments associated with a particle of density 1.16-1.20 g/ml. The serum proteinase which is inhibited by phenylmethanesulfonyl fluoride, but not by sulfhydryl reagents, is presumably a serine proteinase.