Summary
Orgaran is a LMW heparinoid composed of heparan sulphate (83% wlw) of which 4-5% has
high affinity for antithrombin, dermatan sulphate (12% w/w) and chondroitin sulphate
(59 wlw). To examine the contribution of the low-affinity fraction to Orgaran’s antithrombotic
activity we have quantitated the binding of plasma proteins to Orgaran and its component
fractions in whole, hirudin-anticoagulated human plasma. Antithrombin, largely bound
to the high-affinity fraction, and histidine-rich glycoprotein, interacting with low-affinity
components, were the dominant proteins bound to Orgaran. Vitronectin, fibrinogen,
fibronectin, heparin cofactor 11, and apolipoprotein B were also detected in small
amounts. The ratio of bound antithrombin, histidine- rich glycoprotein and vitronectin
to GAG was negatively correlated with the Orgaran concentration in plasma, implying
that the efficacy of Orgaran may not be linearly related to dose. Binding of antithrombin
to the high-affinity fraction was not decreased by other plasma proteins or affected
by addition of low-affinity material. Moreover, the antithrombin and anti-factor Xa
activities of the high-affinity material were unaltered by low-affinity GAGs. On the
basis of our results we conclude that the low-affinity material does not contribute
to the antithrombotic activity of Orgaran by binding non-anticoagulant plasma proteins
and releasing the high-affinity chains to interact with antithrombin and its target
proteinases.
