In the breakdown of fluid phase C3 in plasma, the process of conversion of iC3b to C3c and ‘C3d’ has not been elucidated. Using fluorescent labeled iC3b as a substrate, urokinase(UK) treated but not normal plasma was found to exhibit effective ‘C3d’ production. To address the relationship between the fibrinolytic activity specific for UK-activated plasma and this ‘C3d’ production, an assay system was developed that permitted the simultaneous determination of both activities. This method indicated that ‘C3d’ generation paralleled that of plasmin-dependent fibrinogen degradation. A Km of iC3b for plasmin was 3.0 X 10-6 mol/1 which is similar to the Km of fibrinogen. Plasmin cleavage of iC3b gave rise to C3d] (Mr 42,000) and C3d2 (Mr 28,000). Purified plasmin C3dj showed the same Mr and pi as C3d,g prepared from iC3b, by H and I. C3d2 was derived from C3di. From these in vitro experiments with urokinase-treated plasma, we conclude that in parallel with fibrinolysis efficient cleavage of fluid phase iC3b to C3c + C3d,g and C3d occurs and we hypothesize that this is one mechanism for the generation of C3d in vivo.

Keywords:
Plasmin, Fibrinolysis, C3d-generation, C3d,g, Fluorometry
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1985
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