RELATION OF A β1-GLYCOPROTEIN OF HUMAN SERUM TO THE COMPLEMENT SYSTEM

The protein of human serum, tentatively designated β_1C-globulin, was shown to possess serological activity and to be related to the complement system. Another serum protein (β_1A-globulin) was identified as the inactivated form of β_1C-globulin. Incubation of fresh serum with various immune precipitates or with soluble γ-globulin aggregates at 37°C. resulted in the removal of β_1C-globulin. Treatment of fresh serum with zymosan at 17 and 37°C. had a similar effect. In both instances β_1C-globulin was removed from serum, apparently by conversion to β_1A-globulin. However, isolated β_1C-globulin did not react with immune precipitates or zymosan, nor did β_1C-globulin of serum previously heated at 56°C. Highly purified β_1C-globulin was tested for complement component activity by means of the usual reagents. All of the preparations examined were found to reconstitute the hemolytic activity of guinea pig R₃. However, they failed to reconstitute R₃ obtained from human serum. Isolated β_1A-globulin was found to be inactive in all systems. When isolated β_1C-globulin in either phosphate or in borate buffer was stored at 37°C., the activity detected by means of guinea pig R₃ declined within 6 days to 20 to 30 per cent of its original value. As the activity decreased, β_1C-globulin was gradually converted to β_1A-globulin. Addition of β_1C-globulin to a limited complement system (human C') caused an increase of both initial velocity and final degree of hemolysis. Although β_1C-globulin did not cause lysis of EAC'_{1, 4, 2}, it fully prevented the otherwise rapid decay of EAC'_{1, 4, 2} at 37°C., and so presumably interacted with this complex.