Regulatory system of guinea-pig complement C3b: two factor I-cofactor proteins on guinea-pig peritoneal granulocytes.

Complement factor I is a plasma protease serving for proteolytic inactivation of C3b together with its cofactor. We have identified two factor I-cofactor activities in solubilized extracts of guinea-pig peritoneal granulocytes using guinea-pig factor I (Igp) and fluorescent-labeled methylamine-treated guinea-pig C3 (f-C3(MA)gp). One of these eluted from a chromatofocusing column between pH 7.6-7.1, and the other at about pH 5.7. These two cofactor fractions both interacted with Igp and, to a lesser degree, with human factor I (Ihu) on C3(MA)gp cleaving it into an inactive C3bi analogue, but did not cleave methylamine-treated human C3 (C3(MA)hu) together with Igp or Ihu. These factors are therefore species specific. The neutral and acidic fractions with cofactor activity contained C3(MA)gp-binding proteins with a doublet of 55 kDa and 42 kDa, and a singlet of 160 kDa, respectively, on SDS-PAGE. These proteins may be membrane cofactor protein (MCP) and C3b/C4b receptor (CR1) of guinea-pigs.