Cleavage of the third complement component (C3) and generation of the spasmogenic peptide, C3a, in human serum via the properdin pathway: demonstration of inhibitory as well as enhancing effects of epsilon-amino-caproic acid.

Human and guinea pig serum loses hemolytic activity of the third complement component (C3) during incubation at 37 degrees C. The loss is due to specific C3 cleavage involving the properdin system. This is concluded from the finding that C3 inactivation is prevented by EDTA, by elimination of properdin factor B, and unimpaired in C4 deficient guinea pig serum. In the presence of 1 M epsilon-amino-caproic acid (EACA) spontaneous C3 cleavage is considerably enhanced and accompanied by the appearance of biologically active C3a. Lower concentrations of EACA inhibit rather than enhance C3 cleavage in serum. The inhibitory effect of EACA is due to interference with the interaction of the properdin factors and their action on C3 as demonstrated in various systems: the assembly of an active C3-cleaving complex on zymosan, its regeneration after decay by factor D and factor B (GBG), cleavage of GBG by C3b and factor D in systems of purified components, and cleavage of C3 by preformed properdin complexes. Reactions involving the cobra venom factor were likewise depressed. In all these systems EACA was inhibitory even at 1M concentration. No single step in the development or action of an active properdin system was found to be enhanced by 1 M EACA. The enhancing effect of high concentrations of EACA on C3 cleavage in serum may be explained by its observed inhibition of C3b inactivator (C3bINA). This factor controls the properdin system by destroying C3b. In serum the inhibitory effect of 1 M EACA on C3bINA appears to allow escape of the properdin system from its control and thus to increase its net activity toward C3 despite inhibition of the enzymic reactions proper. At lower concentrations the effect of EACA on C3bINA is apparently less significant; therefore, at low concentrations of EACA, its inhibitory effects on C3 cleavage by the properdin system in serum prediominate.