Changes in the immunochemical properties of highly purified properdin in human serum.

The fate of highly purified properdin (P) upon introduction into normal human serum or properdin-depleted serum (RP) was investigated. It was observed that, concomitant with the activation of the alternate pathway components, properdin underwent immunochemical alterations characterized by a shift in mobility from gamma2 to beta2 position and by an increase in the sedimentation rate from 5.1S to between 6.8 and 9.3S. The immunoelectrophoretic behavior of C3 was also altered with the appearance of a beta2 arc in addition to the beta1C arc. The immunochemical properties of altered P resemble those of "native" properdin in fresh serum. The principle in serum (designated factor F) mediating these changes is a euglobulin with an approximate sedimentation rate and molecular weight of 9.0S and 250,000 daltons, respectively. The alteration in the immunochemical properties of P may be due to aggregation of P molecules or a complex formation between P and a serum euglobulin (probably C3) mediated by factor F and it is associated with loss of ability of P in initiate the alternate pathway of complement activation upon interaction with serum.