On the affinity between the plasminogen activator inhibitor type 2 and apolipoprotein A1

The plasminogen activator inhibitor type 2 (PAI-2) is present in its high molecular weight, glycosylated form in pregnancy plasma. When the protein was purified from retroplacental blood by immunoaffinity chromatography on a PAI-2 antibody column and the retained material was further fractionated by gel filtration chromatography, it was always contaminated by apolipoprotein A1, the latter protein being identified by its N-terminal sequence, molecular weight in SDS-PAGE and immunological properties. The co-purification of the two proteins seemed to indicate a strong affinity between them, suggesting apolipoprotein A1 to be a carrier protein for this PAI-2 form. Further investigation to check this hypothesis showed that the binding of apolipoprotein A1 to the immunoaffinity support was PAI-2-independent and caused by a general surface affinity. This finding was corroborated by a study of the microtitre plate binding properties of the proteins. Pure, high molecular weight PAI 2 did not bind to apolipoprotein A1-coated wells, but the latter protein bound to coated as well as to uncoated wells. Thus, there is no evidence for a specific binding between the two proteins.