Inhibition of heparin activity in plasma by soluble fibrin: evidence for ternary thrombin-fibrin-heparin complex formation.
The ability of heparin to dramatically enhance the inactivation of thrombin (IIa) by antithrombin III (ATIII) in buffer is negated through formation of a IIa-fibrin-heparin ternary complex (Hogg and Jackson, Proc Natl Acad Sci USA 86:3619, 1989; Hogg and Jackson, J Biol Chem 265:241, 1990). IIa, in this ternary complex, is protected from inactivation by ATIII. Our aim was to determine whether fibrin also compromises heparin efficacy in plasma. We found that soluble fibrin ablated the heparin-mediated prolongation of the thrombin time with half-maximal effect at 60 nmol/L fibrin. The heparin-mediated prolongation of the activated partial thromboplastin time (APTT) was also reduced by fibrin with half-maximal effects at 140 nmol/L fibrin using 0.12 U/mL heparin and 500 nmol/L fibrin using 0.25 U/mL heparin. The mechanism of inhibition of heparin activity by fibrin in plasma was determined by measuring IIa-ATIII complexes by enzyme-linked immunosorbent assay (ELISA). Fibrin was found to inhibit the heparin-catalyzed inactivation of IIa by ATIII with half-maximal effect at 97 +/- 19 nmol/L fibrin. Fibrin had no effect on the heparin-catalyzed inactivation of factor Xa by ATIII in plasma, using either standard heparin, a heparinoid preparation (Orgaran; Organon, Lane Cove, Sydney, Australia), or low-molecular weight heparin. These findings imply that fibrin is a potent modulator of heparin activity in vivo by inhibiting heparin-catalyzed IIa-ATIII complex formation through formation of ternary IIa-fibrin-heparin complexes.
Centre for Thrombosis and Vascular Research, School of Pathology, University of New South Wales, S[dney, Australia.
K A
KA
Hotchkiss
C N
CN
Chesterman
P J
PJ
Hogg
D000595:Amino Acid Sequence; D000990:Antithrombin III / Q000378:metabolism; D015951:Factor Xa / Q000378:metabolism; D005337:Fibrin / Q000378:metabolism / Q000494:pharmacology*; D006493:Heparin / Q000378:metabolism* / Q000494:pharmacology; D006494:Heparin Antagonists / Q000494:pharmacology*; D006801:Humans; D008969:Molecular Sequence Data; D010314:Partial Thromboplastin Time; D013917:Thrombin / Q000378:metabolism*; D013918:Thrombin Time
D006494:Heparin Antagonists; D000990:Antithrombin III; D005337:Fibrin; D006493:Heparin; D013917:Thrombin; D015951:Factor Xa