Effect ofa pentosan polysulphate upon thrombin andfactor Xainactivation by antithrombin III

conditions as a function oftheconcentration ofpentosanpolysulphate [afully sulphated (#l-4)-linked D-xylopyranose witha single laterally positioned 4-O-methyl-c-D-glucuronic acid]. Double-reciprocal plots oftheobserved first-order rateconstant against concentration ofpentosan polysulphate gavestraight lines, intercepts on theaxesgiving values formaximumincrease insecond-order rateconstant (bycalculation) and apparentdissociation constant. Thesevalues were: forhumana-thrombin 1.52x107M1-min-1 and3.6pmrespectively, forbovinect-thrombin 6.56x 106MwImin-and0.16 M andforfactor Xa6.86x106w mImin-1and20M.In thepresenceofpentosan polysulphate thedissociation constant fortheinitial complex ofantithrombin IIIandthrombin was showntobereduced fromapprox. 2x 10-3Mto61x 10-6Mwithout apparent change inthelimiting rateconstant of 750min-'. An oligosaccharide (primarily 8-10saccharide units) prepared from heparin andwithhighaffinity forantithrombin IIIbutlowpotency inthethrombinantithrombin IIIinteraction didnotdiminish therateofinteraction catalysed by pentosan polysulphate. Thecatalysis was showntobeduetoa weakelectrostatic interaction, since itwascompletely reversed byconcentrations ofNaClgreater than 0.3 M. Itisconcluded that themechanism isindependent oftheheparin high-affinity binding site on antithrombin IIIandisprobably duetobinding ofthehigh-chargedensity polysaccharide totheproteinase. Itiscalculated thattheacceleration inrate achieved, although lowerthanthatofheparin, approaches thatrequired tobeof physiological significance andmay beofimportance intheanticoagulation role of antithrombin IIIatsites ofhighcharge density whichmay occurinvivo. Heparin functions asananticoagulant primarily byaccelerating thevelocity atwhichantithrombin IIIneutralizes serine proteinases ofthehaemostatic mechanism (Damus etal., 1973). A number of phenomena areimportant inthis catalysis, buta unique feature isanalteration intheconformation oftheantithrombin IIIuponbinding toaspecific domain ontheheparin molecule, whichincreases thereactivity oftheinhibitor asapseudosubstrate (Lindahl etal., 1980, 1982; Casuetal., 1981; Bjork & Lindahl, 1982). A numberofother sulphated polysaccharides