SummaryC1-inhibitor (C1Inh), antithrombin III (ATIII), α1-antitrypsin (a1AT), and α2-antiplasmin (a2AP) are known inhibitors of factor XIa (FXIa). However, their precise contribution to FXIa inactivation in vivo is not known. We investigated FXIa inactivation in chimpanzees and assessed the contribution of these inhibitors to FXIa inactivation in patients with presumed FXI activation.Chimpanzees were infused with FXIa and the various FXIa-FXIa inhibitor complexes formed were measured. Most of FXIa was complexed to C1Inh (68%), followed by a2AP (13%), a1AT (10%), and ATIII (9%). Analysis of the plasma elimination kinetics revealed a half-life time of clearance (t1/2) for the FXIa-FXIa inhibitor complexes of 95 to 104 min, except for FXIa-a1AT, which had a t1/2 of 349 min. Due to this long t1/2, FXIa-a1AT complexes were predicted to show the highest levels in plasma samples from patients with activation of FXI. This was indeed shown in patients with disseminated intravascular coagulation, recent myocardial infarction or unstable angina pectoris. We conclude from this study that in vivo C1Inh is the predominant inhibitor of FXIa, but that FXIa-a1 AT complexes due to their relatively long t1/2 may be the best parameter to assess FXI activation in clinical samples.

C1-inhibitor is the only known inhibitor of the classical pathway of complement and the major inhibitor of the contact pathway of coagulation. Like other serine proteinase inhibitors, C1-inhibitor can exist in three conformations, ie, the native, the proteinase-complexed, and the proteolytically inactivated form. Here we studied the plasma elimination kinetics of these three forms of human C1-inhibitor in rats. The clearance of the complexed form of C1-inhibitor appeared to be the most rapid and depended in part on the proteinase involved (observed plasma t1/2 was 20 minutes for C1s-C1-inhibitor, 32 minutes for kallikrein-C1-inhibitor, and 47 minutes for beta XIIa-C1- inhibitor), whereas that of native C1-inhibitor was the slowest (observed plasma t1/2 4.5 hours). Inactivated C1-inhibitor was cleared with an apparent plasma t1/2 of 1.6 hours. Thus, the short plasma t1/2 of complexed relative to native C1-inhibitor explains why in patients only low concentrations of C1-inhibitor complexes may be observed despite activation of the contact and/or complement systems.

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In the present study we investigated the mechanism of prolongation of the plasma retention of free hemoglobin by polymerization. Polymerization of intramolecularly crosslinked hemoglobin with glutaraldehyde yields a mixture of large polymers, small polymers and monomers. In exchange transfusion experiments in rats we analyzed plasma samples by gel filtration to determine the clearance of polymers of different size. A positive correlation was found between polymer size and vascular retention. Furthermore, the clearance of large polymers appeared to be highly dose-dependent: after 20% and 70% exchange transfusions, we observed for large polymers a plasma half-life of 12 and 26 hours, respectively, whereas the half-life for 64 kD monomers was 4 hours in both cases. The degradation of hemoglobin was followed by measuring the bilirubin excretion. The infused heme was recovered as bilirubin within 72 hours. The delay between the disappearance of free hemoglobin from the plasma and the recovery as bilirubin was about six hours and was not affected by polymerization or dose. We conclude that polymerization prevents the operation of certain clearance mechanisms, while still allowing a route of clearance that is easily saturated. The intracellular degradation of heme into bilirubin is not affected by the modifications of hemoglobin and is not easily saturated.