Kinetic Studies of the Formation of the Properdin System Enzymes on Zymosan: Evidence That Nascent C3b Controls the Rate of Assembly

Abstract

On treatment of guinea pig serum with zymosan (Z) the properdin system becomes activated and combines with Z, yielding a complex ZX that comprises two multi-unit enzymes capable of cleaving complement components C3 and C5, respectively. One of the constitutents of both of these enzymes is complement fragment C3b. The present kinetic study deals with the role of this fragment in the assembly of ZX. As shown earlier, the formation of ZX is much slower in C4-deficient than in normal guinea pig serum. However, if Z coated with C3b (ZC3b) is used, instead of plain Z, the rate of formation of ZX in C4-deficient serum is as great as that in normal guinea pig serum. Hence, Z-bound C3b accelerates assembly of the properdin system enzymes, an observation which is compatible with the view that C3b serves to activate factor D of the properdin system. In addition, the possibility was raised that bound C3b may play a part in the properdin system beyond that of an activator for factor D.