Alterations in the structure of apolipoprotein B-100 determine the behaviour of LDL towards thromboplastin.
Apolipoprotein B-100 acts as an inhibitor of thromboplastin activity independently of the tissue factor pathway inhibitor (TFPI) associated with plasma lipoproteins. Analysis of the primary structure of Apo B-100 showed a higher than expected occurrence of lysine groups in the receptor-binding region. In order to demonstrate the participation of lysine groups of Apo B-100 in the inhibition of thromboplastin, thromboplastin 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 presence of poly-L-lysine. Poly-L-arginine was found to be less effective and neither lysine or arginine monomers had any significant effect on the inhibitory effect of Apo B-100. Alterations in the structure of Apo B-100 reconstituted in lipid vesicles resembling LDL, brought about by lipid peroxidation and lipid loading were examined by means of Fourier transform infra-red spectroscopy. It was found that, upon oxidation without the addition of cupric ions, the apolipoprotein attains a more exposed conformation with an increase in alpha-helical structure. This increase occurred at the expense of beta-structure. On lipid loading, an increase in beta-structure at the expense of the alpha-helix, was demonstrated. It is therefore proposed that the variable action of LDL towards thromboplastin derives from alterations in the secondary structure of the Apo B-100, particularly the receptor-binding region.
Ettelaie
C
C
Department of Biochemistry and Molecular Biology, Royal Free Hospital Medical School, London, UK. Camille@RFHSM.AC.UK
Haris
P I
PI
James
N J
NJ
Wilbourn
B
B
Adam
J M
JM
Bruckdorfer
K R
KR
D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D008077:Lipoproteins, LDL / Q000737:chemistry / Q000378:metabolism*; D008239:Lysine / Q000737:chemistry; D015394:Molecular Structure; D011487:Protein Conformation; D017550:Spectroscopy, Fourier Transform Infrared; D013925:Thromboplastin / Q000378:metabolism*
D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D008077:Lipoproteins, LDL; D013925:Thromboplastin; D008239:Lysine