Purification of a plasma protein that inhibits complement-mediated prevention of immune precipitation

A pre-albumin glycoprotein that inhibits complement-mediated prevention of immune precipitation (PIP) has been purified from normal human serum by sequential affinity chromatography on IgG-Sepharose, protein A-Sepharose and Con A-Sepharose. A total of 4.7 mg of this protein were obtained from 50 ml of serum, representing a yield of 49% and a 253-fold degree of purification. We have named this glycoprotein gp60 as it has an apparent molecular weight of 60,000 on SDS-PAGE. The addition of gp60 to normal serum produced dose-dependent inhibition of both PIP and solubilization of immune precipitates. Maximum inhibition of PIP was achieved by a concentration of 600 micrograms/ml. A monospecific antiserum was produced by the immunization of rabbits, which enabled us to develop an enzyme-linked immunosorbent assay to measure serum concentrations of gp60. In 12 normal sera the mean concentration was 205 micrograms/ml (range 132-258 micrograms/ml), while that in 15 rheumatoid arthritis sera was 515 micrograms/ml (range 430-708 micrograms/ml). The serum concentration of this protein correlated with the level of inhibition of PIP (r = 0.91, P less than 0.01).