RELATION OF A β(1)-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 R(3). However, they failed to reconstitute R(3) 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 R(3) 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.