The activity in human serum which is responsible for conversion of C3 proactivator (C3PA) to C3 activator was shown to reside in a 3S α-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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