An acquired antithrombin autoantibody directed toward the catalytic center of the enzyme.
Antibody inhibitors against human thrombin are rare and have remained poorly characterized. We report the case of a 40-yr-old patient who developed a potent thrombin inhibitor revealed by mild bleeding symptoms and marked prolongation of most laboratory clotting times. After two years of evolution, he died from cerebral hemorrhage. The inhibitor, a polyclonal IgG, was associated with hematological and immunological criteria of autoimmune disorder. Antithrombin IgG was isolated from the patient's plasma by protein A- and thrombin-affinity chromatography. Fab fragments inhibited amidolytic activity of alpha thrombin, and thrombin-thrombomodulin catalyzed protein C activation with a Ki of approximately 10(-8) M in a noncompetitive manner. Alpha to gamma conversion of thrombin resulted in a moderate loss of affinity for the inhibitor. Upon complex formation of thrombin with staphylocoagulase or alpha 2-macroglobulin (alpha 2M), inhibition was decreased by two orders of magnitude and acquired an apparent competitive character. In Western blot experiments, the antibody reacted with active alpha-thrombin, did not react with chloromethylketone-inhibited thrombin and reacted with a lower affinity with iPr2P-thrombin. The inhibitor did not block thrombin binding to benzamidine-, heparin-, or fibrin-Sepharose, but displaced proflavin from its complex with thrombin. Taken together, these results indicate that the patient's autoantibody recognized a conformational structure which includes, at least in part, the apolar binding site adjacent to the catalytic site of thrombin.
Sié
P
P
Laboratoire Hemostase, Centre Hospitalier Universitaire Purpan, Toulouse, France.
Bezeaud
A
A
Dupouy
D
D
Archipoff
G
G
Freyssinet
J M
JM
Dugoujon
J M
JM
Serre
G
G
Guillin
M C
MC
Boneu
B
B
D000328:Adult; D001323:Autoantibodies / Q000276:immunology*; D001665:Binding Sites; D001778:Blood Coagulation Disorders / Q000209:etiology; D000939:Epitopes / Q000032:analysis; D006801:Humans; D007140:Immunoglobulin Fab Fragments / Q000276:immunology; D007074:Immunoglobulin G / Q000145:classification / Q000302:isolation & purification; D008297:Male; D011487:Protein Conformation; D012697:Serine Endopeptidases / Q000276:immunology*; D013917:Thrombin / Q000037:antagonists & inhibitors / Q000276:immunology* / Q000378:metabolism
D001323:Autoantibodies; D000939:Epitopes; D007140:Immunoglobulin Fab Fragments; D007074:Immunoglobulin G; D012697:Serine Endopeptidases; D013917:Thrombin