A new protein named Ss-bp, or C4-bp, with specific binding affinity for activated C4 has been found in mouse serum. Ss-bp is a heat-stable (60° C, 1 hr) β-globulin with a sedimentation coefficient of 10S. Ss and Ss-bp are separated by filtration of EDTA-plasma in Sephadex G-200. However, in serum, Ss-bp binds tightly to Ss and the complex is found in the excluded volume of the column. After electrophoresis in agarose at pH 8.6, the Ss-bp/Ss complex moves faster toward the anode than either protein alone. For this reason Ss-bp may appear to be polymorphic in serum of mice with high levels of C4 (Ss-H) versus those with low levels of C4 (Ss-L) (Ferreira, Takahashi and Nussenzweig, J. Exp. Med., in press).
Ss-bp levels are higher in serum of males (160 µg/ml) than in female mice (60 µg/ml). As in the case of other mouse C components (C4, C5), the concentration of Ss-bp in serum is markedly influenced by testosterone.