ZINC REGULATES THE FUNCTION OF 2 SUPERANTIGENS

Staphylococcal enterotoxins bind with high affinity to class II major histocompatibility complex proteins and subsequently stimulate large numbers of T cells via the V-beta portion of the T-cell receptor. Binding of enterotoxin A and enterotoxin E to HLA-DR was completely abolished by low levels of EDTA, whereas binding of toxic shock toxin was unaffected. Addition of Zn2+ to as little as 2-mu-M excess over EDTA completely reconstituted binding, but Ca2+, Mg2+, Cu2+, Fe2+, and Mn2+ had no effect. The dissociation constant (K(d)) of Zn-65(2+) binding to a single site on purified enterotoxin A was 2-mu-M, and addition of purified HLA-DRI did not alter the K(d), indicating that the binding site was exclusive to enterotoxin A. In the presence of saturating levels of zinc the K(d) for enterotoxin A binding to purified HLA-DRI was 25 nM. Thus, zinc binding is an essential first step in the formation of the major histocompatibility complex binding domain of at least two bacterial superantigens. Given the measured K(d) of zinc binding to enterotoxin A, serum levels of free zinc (0.2-1.0-mu-M) may well regulate the toxic sequelae by these two superantigens.