ON THE AFFINITY BETWEEN THE PLASMINOGEN-ACTIVATOR INHIBITOR TYPE-2 AND APOLIPOPROTEIN A(1)

The plasminogen activator inhibitor type 2 (PAI-2) is present in its h igh molecular weight, glycosylated form in pregnancy plasma. When the protein was purified from retroplacental blood by immunoaffinity chrom atography on a PAI-2 antibody column and the retained material was fur ther fractionated by gel filtration chromatography, it was always cont aminated by apolipoprotein A(1), the latter protein being identified b y its N-terminal sequence, molecular weight in SDS-PAGE and immunologi cal properties. The co purification of the two proteins seemed to indi cate a strong affinity between them, suggesting apolipoprotein A(1) to be a carrier protein for this PAI-2 form. Further investigation to ch eck this hypothesis showed that the binding of apolipoprotein A(1) to the immunoaffinity support was PAI-2-independent and caused by a gener al surface affinity. This finding was corroborated by a study of the m icrotitre plate binding properties of the proteins. Pure, high molecul ar weight PAI 2 did not bind to apolipoprotein A(1)-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.