Interaction of Zymosan and of Activated Properdin with Factor {ie402-1}-Depleted Guinea Pig Serum: Implications for the Mechanism of Initial C 3 Cleavage via the Alternative Complement Pathway

Abstract

Factor {ie402-2} of the alternative C pathway was specifically removed from guinea pig serum. The resulting serum reagent (RD) supported neither the formation of a C 3-cleaving enzyme on zymosan (Z) nor the inactivation of C 3 or of factor B in the presence of Z or activated properdin ({ie402-3}). Addition of purified {ie402-4} to RD restored these properties. Studies where limited amounts of purified {ie402-5} were added to RD with Z or {ie402-6} as activating substances, gave the following results: (1) Inactivation ofB and ofC3 occurs in the presence of minute amounts of {ie402-1}. (2) C 3 inactivation is more efficient than B inactivation and proceeds even in the absence of detectable enzymatic B activation. (3) C 3 cleavage at any {ie402-8} concentration tested is always accompanied by uptake of C3 fragments onto Z. With respect to initial C 3 cleavage via the alternative C pathway these data suggest that the initiating reaction is {ie402-9}-dependent, very efficient in depositing C 3 fragments on particulate activating substances such as Z and able to utilize factor B in an apparently uncleaved form.