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 (ß_1E-globulin). Inactivation is accompanied by changes in electrophoretic and ultracentrifugal properties of ß_1E-globulin. If non-sensitized sheep erythrocytes are present during the action of C'1 esterase on ß_1E-globulin, a complex is formed consisting of cells and cytolytically active fourth component (EC'4). Thus, inactivation of ß_1E-globulin by C'1 esterase appears to be preceded by a state of activation enabling ß_1E-molecules to combine with cell membrane receptors. Acceptor groups appear to be present also in 7S γ-globulin and in ß_1E-globulin itself, since C'1 esterase can induce the formation of ß-ß and of ß_1E-7S γ-globulin complexes.