Binding of α2-I variants to monoclonal antibody 12F1. The α2 I proteins were incubated with immobilized monoclonal antibody 12F1. Bound protein was determined by ELISA as described in “Materials and methods.” The figure shows the mutant proteins (Y285F/E318W, ▴; Y285F, ▪) that had a markedly different binding curve than the wild type (♦). Each point represents the mean ± SD of values obtained in 2 independent sets of triplicate assays.