Binding of α2-macroglobulin and haptoglobin to Actinomyces pyogenes
All 25 cultures of Actinomyces pyogenes tested in the present study bound 125I-labelled human α2-macroglobulin with a mean binding of 65.6%. Thirteen cultures also bound 125I-labelled human haptoglobin with a mean of 51.5%. None interacted with fibrinogen, fibronectin, immunoglobulin G, or albumin. Twenty-eight cultures representing other species of actinomycetaceae did not show any interaction with α2-macroglobulin, haptoglobin, and other plasma proteins tested. The binding of α2-macroglobulin and haptoglobin to A. pyogenes was saturable and could be completely inhibited by the respective unlabelled plasma proteins. The binding of α2-macroglobulin could not be inhibited by unlabelled haptoglobin. On the other hand, α2-macroglóbulin blocked the binding of haptoglobin, possibly by steric hindrance. Treatment of the bacteria with trypsin reduced their binding activities for α2-macroglobulin and haptoglobin indicating the protein nature of the binding sites. Exposure to heat (1 h, 80 °C) significantly diminished the binding activity for haptoglobin, but not that for α2-macroglobulin. The binding of α2-macroglobulin and haptoglobin could be an important feature in the classification of A. pyogenes among the members of actinomycetaceae.