Generation of C3d,g and C3d by urokinase-treated plasma in association with fibrinolysis

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 paralled that of plasmin-dependent fibrinogen degradation. A Km of iC3b for plasmin was 3.0 x 10^-6 mol/l which is similar to the Km of fibrinogen. Plasmin cleavage of iC3b gave rise to C3d₁ (M(r) 42,000) and C3d₂ (M(r) 28,000). Purified plasmin C3d₁ showed the same M(r) and pI as C3d,g prepared from iC3b, by H and I. C3d₂ was derived from C3d₁. 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.