Variance in the activity of the fourth component of mouse complement.

There are two methods for hemolytic titration of C4. One is a method using EAClgp, oxyC2hu and EDTA-Cgp (E-C method) and the other using EAClgp, oxyC2hu and C4Dgp (C4D method). The C4 titers obtained by these two methods differed depending on whether the C4 exists in plasma or in serum. In the case of Ssh strain, serum C4 activity assayed by the E-C method was higher than that by the C4D method, while plasma C4 activity assayed by the E-C method was lower than that by the C4D method. It was hypothesized that these results would occur from the difference in the state of C4 molecule present in serum or plasma. From chromatographic analysis, it was apparent that the C4 molecule in serum existed in at least four different molecular states; free C4 molecule with low Ss antigenicity, hemolytically inactive Ss protein, active C4 with high molecular weight and low Ss antigenicity, and inactive Ss protein with high molecular weight. Their expression of Ss antigen and the hemolytic C4 activity were different, and each type of C4 molecule behaved differently in the assay of the two methods. Furthermore an important role of mouse C5 in the hemolytic assay of mouse C4 is discussed.