Endogenous Inhibitors of Factor B

Proteins which were able to bind noncovalently with mouse factor B were found in cells that are nonsecretors of factor B such as mouse‐established monocytic cells and L cells but not in peritoneal resident macrophages. These proteins were isolated from lysates of L cells and separated into four distinct proteins by preparative SDS‐polyacrylamide gel electrophoresis, with molecular weights of 25K, 28K, 33K, and 35K. The individual proteins formed a complex with purified mouse factor B at a molecular ratio of 1: 1 and inhibited its hemolytic activity. Proteins 25K and 28K inhibited the hemolytic activity of an activated form of factor B combined with cobra venom factor as well as that of the native form. These inhibitors did not affect the hemolytic activity of the second component of complement in mouse serum. The inhibitory activity of the 25K protein was partially inhibited by antiserum raised against it in rabbits.