THE MECHANISM OF INHIBITION OF FACTOR-III (THROMBOPLASTIN) ACTIVITY BY APOLIPOPROTEIN-B-100 - PROTEIN-PROTEIN INTERACTIONS

Factor III (thromboplastin) activity is inhibited by apoB-100, but the mechanism of inhibition is unknown. By examining the effect of purifi ed apoB-100 on factor III activity, we showed that apoB-100 can inhibi t factor III via a different mechanism from that caused by the tissue- factor pathway-inhibitor, which is mainly carried on the surface of li poproteins. Although the presence of calcium ions and factors X and VI I may enhance the rate of inhibition, they are not a prerequisite for the inhibition of factor III by apoB-100. In addition, by investigatin g the changes in the UV spectra of apoB-100 on interaction with factor III and factors X and VII and by assigning the shifts in absorption s pectra to particular amino acids, we showed that these interactions in volve negative and positive residues within these proteins. By followi ng the rates of interactions between apoB-100 and either factors III, X, or VII, a two-step mechanism for the inhibition process involving f actors X and VII was postulated. In this mechanism, the primary intera ction of apoB-100 with factor III is followed by a rate-limiting step that can be accelerated by the presence of either factor X or VII and leads to the inhibition of factor III. Furthermore, a computer-based a nalysis of the sequences of factor III revealed a possible binding sit e for apoB-100.