Nephritic factor: its structure and function and its relationship to initiating factor of the alternative pathway.

Nephritic factor (NF) has a molecular weight of 170,000 and is composed of two disulfide-linked 85,000-dalton chains. NF assembles the fluid phase C3 convertase from Factors B and D, C3, and magnesium by physically incorporating itself into the enzyme complex. NF exerts its stabilizing effect on the cell-bound C3/C5 convertase, EC3b,B, by physically associating itself with this complex. On decay of the cell-bound enzyme NF is released into the fluid phase and retains its binding and stabilizing potential. Its activity is resistant to diisopropylfluorophosphate treatment. Because NF causes agglutination of EC3b,B, it must be endowed with more than one binding site.