Further characterization of haptoglobin binding to streptococci of serological group A.

Certain group A streptococci with surface antigen T 4 possess surface receptors for human haptoglobin (Hp). Binding of 125I Hp 2-1 to two representative group A streptococcal cultures could be inhibited by unlabelled Hp 2-1, Hp 2-2 and Hp 1-1 but not by the alpha 1, alpha 2 or beta chains of Hp. Hp complexes formed with equine hemoglobin and asialo-Hp also reduced 125I-Hp 2-1 binding to group A streptococci. Hp binding proteins could be solubilized from streptococcal surface by hot acid treatment of the bacteria and purified by subsequent affinity chromatography on human Hp 2-1 sepharose. The isolated Hp binding proteins specifically inhibited 125I-Hp 2-1 binding to group A streptococci and retained their 125I-Hp 2-1 binding activity in a dot binding assay on nitrocellulose membranes. SDS-PAGE and protein blots of Hp binding proteins developed with 125I-labeled Hp 2-1 revealed numerous high molecular weight proteins with 125I-Hp 2-1 binding activity.