Specific Binding Sites for Monomeric and Aggregated β₂-Microglobulin on Surface of Groups A, B, C, and G Streptococci

Abstract

Human β₂-microglobulin (β₂-m) was isolated from urine samples of patients with tubular dysfunctions and aggregated with glutaraldehyde. Four aggregates with molecular weights of 800, 000, 480, 000, 260, 000, and 60, 000 were separated by filtration on Sephacryl S-300. The aggregates and monomeric β₂-m (11, 800MW) were subsequently labeled with ¹²⁵I and tested for binding to streptococci. Group A streptococci bound only aggregated β₂-m with a mean binding of 44.5%. Most of the group G streptococci, on the other hand, bound only monomeric β₂-m with a mean binding of 58%. Among group B streptococci the serotypes with protein antigens interacted mainly with monomeric β₂-m and those without protein antigens preferentially with aggregated β₂-m. Nontypable group B streptococcal serotypes did not bind monomeric or aggregated β₂-m. Of the streptococci belonging to group C, S. equisimilis reacted with monomeric β₂-m and S. dysgalactiae with aggregated β₂-m. S. equi did not interact with monomeric β₂-m or aggregated β₂-m. Bindings of monomeric β₂-m and aggregated β₂-m were saturable and could be inhibited by the respective unlabeled forms of β₂-m. Fibrinogen, fibronectin, α₂-macroglobulin, haptoglobin, or immunoglobulin G did not inhibit the binding of either form of β₂-m. The binding sites for monomeric β₂-m were more susceptible to trypsin than those for aggregated β₂-m. Treatment of streptococci with pronase destroyed their binding activities for monomeric and aggregated β₂-m. Both monomeric β₂-m and aggregated β₂-m binding sites were sensitive to heat. The Scatchard plots of monomeric β₂-m and aggregated β₂-m were linear with K_d of 1.29×10^-9M and 1.9×10^-9M respectively. The number of binding sites per bacterium were estimated to be 81, 000 for monomeric β₂-m and 1, 210 for aggregated β₂-m.