Covalent complexes between low molecular weight heparin fragments and antithrombin III - inhibition kinetics and turnover parameters.

Two high affinity heparin fragments (Mr 4,300 and Mr 3,200) were covalently coupled to antithrombin III (J. Biol. Chem. 1982; 257: 3401--3408) with an apparent 1:1 stoichiometry and a 30--35% yield. The purified covalent complexes inhibited factor Xa with second order rate constants very similar to those obtained for antithrombin III saturated with these heparin fragments and to that obtained for the covalent complex between antithrombin III and native high affinity heparin. The disappearance rates from plasma in rabbits of both low molecular weight heparin fragments and their complexes could adequately be represented by two-compartment mammillary models. The plasma half-life (t1/2) of both low Mr-heparin fragments was approximately 2.4 hr. Covalent coupling of the fragments to antithrombin III increased this half-life about 3.5 fold (t1/2 congruent to 7.7 hr), approaching that of free antithrombin III (t1/2 congruent to 11 +/- 0.4 hr) and resulting in a 30 fold longer life time of factor Xa inhibitory activity in plasma as compared to that of free intact heparin (t1/2 congruent to 0.25 +/- 0.04 hr).