ANTITHROMBIN III NORTHWICK PARK: CHARACTERIZATION OF AN INACTIVE HIGH MW COMPLEX WITH INCREASED AFFINITY FOR HEPARIN

It has been shown previously that antithrombin III Northwick Park (AT III NWP) has reduced ability to inactivate thrombin and is characterised by an additional anodal component on crossed immunoelectrophoresis. We have applied plasma from an affected family member to heparin-Sepharose and eluted the AT III with a salt gradient. Evidence will be presented that the anodal component has* higher affinity for heparin than normal AT III. Furthermore, this variant component is present in plasma as a MW >120,000 inactive complex whose tryptic peptide FAB map contains numerous signals not characteristic of normal AT . 111 . This complex can be reduced with dithiothreitol to two non identical bands on SDS PAGE with MW ~60,000, only one of which reacts with anti-AT III. Using ion-exchange chromatography and HPLC these two components have been isolated and separated. The N-terminal sequence of the protein that does not react with anti-AT III is believed to be Asp-Ala-His-Ile-Ser-Glu. Structural investigations on the variant AT III are underway.