¹ From the Department of Experimental Pathology, Scripps Clinic and Research Foundation, La Jolla, California 92037

The activity in human serum which is responsible for conversion of C3 proactivator (C3PA) to C3 activator was shown to reside in a 3S -globulin. The factor, called C3PA convertase (C3PAse), was obtained in partially purified form. For conversion of C3PA, C3PAse required participation of metal ions and of a C3 fragment, which in physicochemical and antigenic properties resembled C3b. Isolated, native C3 failed to substitute for the fragment, but did restore the impaired C3 activator system in hydrazine-treated serum. Unlike native C3, the C3 fragment initiated conversion of C3PA in whole serum. A hypothetical concept which envisions the C3 fragment as effector of C3PAse has been proposed.

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