Human lactoferrin (HLf) is an iron-binding protein with antimicrobial activity that is present in high concentrations in milk and various exocrine secretions. HLf is also an acute-phase protein secreted by polymorphonuclear leucocytes, and its binding to a large number of clinical isolates of has been described recently from our laboratory. We have now characterised the HLf-staphylococcal interaction in strain MAS-89. The binding of I-HLf to strain MAS-89 reached saturation in <90 min and was maximal between pH 4 and 9. Unlabelled HLf displaced I-HLf binding. Various plasma and subepithelial matrix proteins, such as IgG, fibrinogen, fibronectin, collagen and laminin, which are known to interact specifically with , did not interfere with HLf binding. A Scatchard plot was non-linear; this implied a low affinity (1‡55 × 10 L/mol) and a high affinity (2‡70 × 10 L/mol) binding mechanism. We estimated that there were 5700 HLf binding sites/cell. The staphylococcal HLf-binding protein (HLf-BP) was partially susceptible to proteolytic enzymes or periodate treatment and was resistant to glycosidases. An active HLf-BP with an apparent M of 450 Kda was isolated from strain MAS-89 cell lysate by ion-exchange chromatography on Q-sepharose. In SDS-PAGE, the reduced HLf-BP was resolved into two components of 67 and 62 Kda. The two components demonstrated a positive reaction with HLf-HRPO in a Western blot. These data establish that there is a specific receptor for HLf in .


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