ISOLATION AND DESCRIPTION OF THE FOURTH COMPONENT OF HUMAN COMPLEMENT.

Purification of the activity of the fourth component of human complement resulted in the isolation of a highly homogeneous serum protein. Since this protein has not been recorded previously it was called beta(1E)-globulin on the basis of its immunoelectrophoretic behavior. C'4 activity and beta(1E)-globulin were found to have highly similar, if not identical physicochemical characteristics. Moreover, beta(1E)-globulin was shown to exhibit the specific behavior of C'4 activity in that it is taken up only by cells which contain activated C'1. DFP-inactivated C'1 failed to catalyze uptake of the protein. Treatment with hydrazine which is known to destroy C'4 activity, led to changes in the physicochemical properties of beta(1E)-globulin and rendered the molecule incapable to combine with C'1-containing cells. The evidence indicates that beta(1E)-globulin represents the fourth component of human complement.