THE ROLE OF HUMAN FACTOR-X ACTIVATION PEPTIDE IN ACTIVATION OF FACTOR-X BY FACTOR IXA

We studied the interaction of factor X activation peptide (XAP) with f actor IXa and factor Xa and the effect of XAP on factor IXa-catalyzed activation of factor X. XAP associated with factor Xa in the presence of 5 mM Ca2+ was dissociated from factor Xa by gel chromatography usin g Ultrogel AcA54 in 5 mM EDTA, or in 8 M urea-0.1% SDS. An exogenous i solated XAP inhibited the factor IXa-catalyzed factor X activation bot h in the presence and absence of factor VIIIa. 4-Amidinophenylmethylsu lfonyl (aPMS)-factor Xa independent of XAP also inhibited the factor X activation more effectively than XAP alone in the presence of factor VIIIa. However, aPMS-factor Xa independent of XAP hardly inhibited the factor X activation in the absence of factor VIIIa. The binding of I- 125-labeled factor X to the aPMS-factor IXa fixed to a microwell plate was inhibited by unlabeled factor X or XAP, but not by aPMS-factor Xa with or without XAP. Factor IXa directly bound to XAP and aPMS-factor Xa with XAP, but did not bind to aPMS-factor Xa without XAP. These fi ndings suggest that the region of XAP in factor X directly interacts w ith factor IXa, and factor Xa region other than XAP interacts with fac tor VIIIa. Desialation or deletion of N-linked carbohydrates of XAP re duced the inhibitory activity of XAP for the factor X activation by fa ctor IXa to approximately 50% of that of the intact XAP. This suggests that the sialic acids in the carbohydrate chains of the XAP region pa rtly contribute to the interaction with factor IXa during its activati on.