Mode of Action of Staphylococcal Leukocidin: Relationship between Binding of 125I-Labeled S and F Components of Leukocidin to Rabbit Polymorphonuclear Leukocytes and Leukocidin Activity

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The binding of 125I-labeled S component to rabbit polymorphonuclear leukocytes was found to be concentration dependent and saturable at 37°C. Scatchard analysis of the binding curve gave a straight line, indicating that S component binds to a single population of sites. The dissociation constant, KD, derived from the Scatchard plot was 5.57 × 10−9 M, and the number of binding sites per leukocyte was calculated to be approximately 5,300. Unlabeled S component (10−8 M) or subunit B of cholera toxin (10−7 M) readily competed with 125I-labeled S component binding, and the labeled S component, preincubated with ganglioside GM1 at equimolar proportions for 5 min, lost the binding capacity to the leukocyte membranes. The binding number of 125I-labeled F component to leukocidin-sensitive cells, such as rabbit polymorphonuclear leukocytes and the established human myelocytic leukemia cells, in the absence and in the presence of the unlabeled S component (2.1 nM), was calculated to be 50 and 1,300 molecules per cell, respectively. This increased binding of the labeled F component was time and temperature dependent. The binding number of labeled F component to other cell types comparatively insensitive to leukocidin, such as erythrocytes, adipocytes, intestinal cells, and HeLa cells, was calculated to be less than 50 molecules per cell in spite of the sufficient amount of unlabeled S component bound to their cells. These observations are consistent with the view that in rabbit leukocyte the S component, preferentially bound to the cell surface at 5,300 molecules per cell, contributes to enhance the F component binding up to about 1,300 molecules per cell and may thus play a role of synergistic action of both leukocidin components on the cell membranes in the leukocytolysis.

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