ALTERATIONS IN THE STRUCTURE OF APOLIPOPROTEIN B-100 DETERMINE THE BEHAVIOR OF LDL TOWARDS THROMBOPLASTIN

Apolipoprotein B-100 acts as an inhibitor of thromboplastin activity i ndependently of the tissue factor pathway inhibitor (TFPI) associated with plasma lipoproteins. Analysis of the primary structure of Apo B-1 00 showed a higher than expected occurrence of lysine groups in the re ceptor-binding region. In order to demonstrate the participation of ly sine groups of Apo B-100 in the inhibition of thromboplastin, thrombop lastin and Apo B-100 were incubated together in the presence of poly-L -lysine, poly-L-arginine, lysine and arginine monomers. The inhibition of thromboplastin by Apo B-100 was completely suppressed in the prese nce of poly-L-lysine. Poly-L-arginine was found to be less effective a nd neither lysine or arginine monomers had any significant effect on t he inhibitory effect of Apo B-100. Alterations in the structure of Apo B-100 reconstituted in lipid vesicles resembling LDL, brought about b y lipid peroxidation and lipid loading were examined by means of Fouri er transform infra-red spectroscopy. It was found that, upon oxidation without the addition of cupric ions, the apolipoprotein attains a mor e exposed conformation with an increase in alpha-helical structure. Th is increase occurred at the expense of beta-structure. On lipid loadin g, an increase in beta-structure at the expense of the alpha-helix, wa s demonstrated. It is therefore proposed that the variable action of L DL towards thromboplastin derives from alterations in the secondary st ructure of the Apo B-100, particularly the receptor-binding region.