Solid phase radioimmunoassay of properdin

This report describes a solid phase radioimmunoassay for measuring properdin concentration, independent of other proteins in the properdin system. Polystyrene tubes are coated with mono-specific rabbit antiserum to human properdin. The competitive binding of increasing concentrations of unlabeled properdin (10–50 ng) and a constant amount of 125I properdin (40 ng) to the antiproperdin-coated tubes is a function of the concentration of the unlabeled properdin. Properdin concentration in unknown samples can therefore be calculated. Optimal conditions for each step of the assay procedure are described. The method is sensitive and reproducible at the nanogram level. Polystryrene tubes coated with antiproperdin serum fail to bind radiolabeled human IgG, C3, or C5. The quantitative validity of the assay has been established by compairing experimental and theoretical values for mixtures of serum and added purified properdin. By this method, sera from 19 normal individuals contained 17.2–29.4 μg/ml, with the mean and standard deviation of the mean at 24.7±3.9 μg/ml.