Ii. Ui~aasrano by C4i Hu O~" C | H" Sp~.cmmizy for C2 Hu*

I t has been previously demonstrated (1) tha t the fluid phase inactivation of human second component of complement (C2) by activated human first component (CI) is markedly enhanced in the presence of homologous fourth component (C4), whereas heterologous C4 is inhibitory 1. This process was found to involve enzymatically active CI and the hemolyticaUy inactive product of its action on C4, C4i. Since it is established that the Cis subunit of the Ci m a c r e molecule is directly capable of altering the electrophoretic mobility and hemolytic activity of C4 (2) and C2 (3, 4), the enzymatic site for C2 must be somewhat masked in the intact Ci molecule. The present studies suggest C4i interacts directly with CI to uncover the active site for C2.