The effect of Ca2+, phospholipid and factor V on the anti-(factor Xa) activity of heparin and its high-affinity oligosaccharides.
The influence of Ca2+, phospholipid and Factor V was determined on the rate of inactivation of Factor Xa by antithrombin III, in the absence and in the presence of unfractionated heparin and of three high-affinity heparin oligosaccharides in the Mr range 1500-6000. In the absence of heparin the addition of Ca2+, phospholipid and Factor V caused a 4-fold decrease in rate of inactivation of Factor Xa. As concentrations of unfractionated heparin were increased the protective effect of Ca2+/phospholipid/Factor V was gradually abolished, and at a concentration of 2.4 nM there were no differences in rates of neutralization of Factor Xa in the presence or absence of Ca2+, phospholipid and Factor V. In contrast, heparin decasaccharide (Mr 3000) and pentasaccharide (Mr 1500) fragments were unable to overcome the protective effect of Ca2+/phospholipid/Factor V; in the presence of these components their catalytic efficiencies were 16-fold and 40-fold less respectively than that of unfractionated heparin. A heparin 20-22-saccharide fragment (Mr approx. 6000) gave similar inactivation rates in the presence and in the absence of Ca2+/phospholipid/Factor V. Human and bovine Factor Xa gave similar results. These results indicate that in the presence of Ca2+/phospholipid/Factor V optimum inhibition of Factor Xa requires a saccharide sequence of heparin additional to that involved in binding to antithrombin III. The use of free enzyme for the assessment of anti-(Factor Xa) activity of low-Mr heparin fractions could give misleading results.
D000990:Antithrombin III / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002384:Catalysis; D005165:Factor V / Q000494:pharmacology*; D005170:Factor X / Q000037:antagonists & inhibitors*; D015951:Factor Xa; D006493:Heparin / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D009844:Oligosaccharides / Q000494:pharmacology; D010743:Phospholipids / Q000494:pharmacology*
D009844:Oligosaccharides; D010743:Phospholipids; D000990:Antithrombin III; D005165:Factor V; D005170:Factor X; D006493:Heparin; D015951:Factor Xa; D002118:Calcium