Relation of a Β1-glycoprotein of Human Serum to the Complement System

The protein of human serum, tentatively designated beta(1C)-globulin, was shown to possess serological activity and to be related to the complement system. Another serum protein (beta(1A)-globulin) was identified as the inactivated form of beta(1C)-globulin. Incubation of fresh serum with various immune precipitates or with soluble gamma-globulin aggregates at 37 degrees C. resulted in the removal of beta(1C)-globulin. Treatment of fresh serum with zymosan at 17 and 37 degrees C. had a similar effect. In both instances beta(1C)-globulin was removed from serum, apparently by conversion to beta(1A)-globulin. However, isolated beta(1C)-globulin did not react with immune precipitates or zymosan, nor did beta(1C)-globulin of serum previously heated at 56 degrees C. Highly purified beta(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 beta(1A)-globulin was found to be inactive in all systems. When isolated beta(1C)-globulin in either phosphate or in borate buffer was stored at 37 degrees 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, beta(1C)-globulin was gradually converted to beta(1A)-globulin. Addition of beta(1C)-globulin to a limited complement system (human C') caused an increase of both initial velocity and final degree of hemolysis. Although beta(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 degrees C., and so presumably interacted with this complex.