C'1 ESTERASE EFFECT ON ACTIVITY AND PHYSICOCHEMICAL PROPERTIES OF THE FOURTH COMPONENT OF COMPLEMENT.

Highly purified C'1 esterase of human serum is capable of inactivating isolated fourth component of human complement (beta(1E)-globulin). Inactivation is accompanied by changes in electrophoretic and ultracentrifugal properties of beta(1E)-globulin. If non-sensitized sheep erythrocytes are present during the action of C'1 esterase on beta(1E)-globulin, a complex is formed consisting of cells and cytolytically active fourth component (EC'4). Thus, inactivation of beta(1E)-globulin by C'1 esterase appears to be preceded by a state of activation enabling beta(1E)-molecules to combine with cell membrane receptors. Acceptor groups appear to be present also in 7S gamma-globulin and in beta(1E)-globulin itself, since C'1 esterase can induce the formation of beta-beta and of beta(1E)-7S gamma-globulin complexes.