Quantitation of human complement fragment C4ai in physiological fluids by competitive inhibition radioimmune assay

A method is described to quantitate complement fragment C4ai in human plasma, synovial fluid, and urine. Samples are first precipitated with 50% saturated (NH4)2SO4 to remove cross-reactive macromolecules C4 and pro-C4. Whereas greater than 97% of C4 is removed by this precipitation step, 88% of C4ai remains in solution. Second, the concentration of C4ai in supernatant fractions is determined by double antibody competitive inhibition radioimmunoassay. C4a was recently completely sequenced (Moon et al., 1981) and is readily available as a pure standard. Examination of the specificity of this method confirmed it was indeed specific for C4a antigenicity. Immunochemically depleted C4-deficient plasma and inulin-activated reconstituted C4-deficient plasma exhibited less than 0.1% of the immunoreactivity of untreated plasma. In addition, good agreement was observed in analyses of aggregated IgG activated serum between the experimentally determined concentration of C4ai and that expected from the initial concentration of C4. As a result, recovery and measurement of C4ai in physiological fluids with this method appear both quantitative and specific. Based on results from 17 adult volunteers, the average concentration of C4ai in normal plasma is 488 ng/ml. Interestingly, significant correlation could not be demonstrated between the levels of C4 and C4ai in normal plasma. The mean concentration of C4ai in human urine is 0.5 ng/ml.