FLUID PHASE DESTRUCTION OF C2^hu BY C1^hu : I. ITS ENHANCEMENT AND INHIBITION BY HOMOLOGOUS AND HETEROLOGOUS C4

The fluid phase inactivation of C2^hu by C1^hu is markedly enhanced by the presence of C4^hu. The enhancement is afforded by C1 inactivated C4^hu, namely C4i^hu, and requires the simultaneous presence of enzymatically active C1. Heterologous C4 of guinea pig origin protects C2^hu from the inactivation by C1^hu. Thus, in both the fluid phase and on the cellular intermediate, C4^hu is essential to the specific action of C1^hu on C2^hu. It is possible that C4i alters C2 so as to present a more suitable substrate to the C1 enzyme or that C4i acts on the C1 to uncover a specificity for native C2.