Properdin (P), a highly basic euglobulin, was purified from human serum to molecular homogeneity without the use of zymosan. Isolated P was found to efficiently initiate activation of the alternate pathway of complement activation (C3 activator or properdin system) and to be an essential component during its early reaction stages. The activity of isolated P did not require the presence of an activating polysaccharide. It was therefore concluded that purified P had been obtained in an activated form (P).

In an isolated reaction system containing purified C3, C3 proactivator (C3PA), and C3 proactivator convertase (C3PAse), P was able to mediate the activation of C3PAse which in turn activated C3PA to cleave C3. This activation of C3PAse was found to depend on the presence of native C3. These results allowed the formulation of a concept in which P is envisaged to act as a modulator of native C3 enabling it to activate C3PAse.

Activation of C3 was efficiently mediated by P in the fluid phase. Efficient activation of C5, however, required the participation of an insoluble polysaccharide (zymosan). The possibility is raised therefore that P might also be an integral part of the multimolecular C5 convertase of the alternate pathway of complement activation.

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