Human complement factor H: an additional gene product of 43 kDa isolated from human plasma shows cofactor activity for the cleavage of the third component of complement.

In addition to the 150-kDa factor H protein, we have previously described a 43-kDa factor H molecule in human plasma, which probably represents a translational product of the additional 1.8-kb mRNA for factor H. This factor H was isolated from human plasma by means of immunoaffinity chromatography and high-performance liquid chromatography. When tested for its functional activity, this purified 43-kDa H protein was shown to act as cofactor for factor I- mediated cleavage of fluid-phase C3b to iC3b.