Binding of human haptoglobin to streptococci of serological group A

All 11 streptococcal cultures of serological group A possessing antigen T4 bound human 125I-labelled haptoglobin (hp) and fibronectin (fbn). Most of these cultures also interacted with alpha 2-macroglobulin (alpha 2m), immunoglobulin G (IgG) and fibrinogen (fib), none with albumin. The binding of the labelled hp could be completely inhibited by unlabelled hp. It was not significantly affected by the other plasma proteins, except alpha 2m. Trypsinization of the streptococci reduced their binding activities for hp, fbn, fib and alpha 2m, but not for IgG. Extraction with guanidine hydrochloride removed non-covalently bound substances from the streptococci without loss in binding-activity for hp.