Thermally induced structural changes in immunoglobulin E.

The ability of immunoglobulin E (IgE) to bind to the cell membrane of basophiles and mast cells is progressively lost when the molecule is heated at 56”. In the present study, we have investigated the structural changes induced by heating under conditions known to destroy this biological function, although direct measurements of cytotropic activity were not made. After 30 min at 56” circular dichroism spectrum of intact IgE showed changes in both the aromatic side chain and peptide bond spectral regions. These effects were only partially reversible upon cooling to 25”. Ultraviolet absorption difference spectra suggested that tyrosine and tryptophan residues were exposed following the heat treatment. The structural effects observed upon heating could be reversed by complete unfolding in guanidine HCl and subsequent refolding upon removal of the guanidine. Parallel studies with proteolytic fragments of IgE (F(ab’)z, Fc”, and PC) indicated that the irreversible thermal effects were restricted to the COOH-terminal region of the heavy (3 chain of IgE. F(ab’)z and Fc” underwent fully reversible conformational changes at 56”, as judged by circular dichroism and difference spectra. The irreversible transition seen in IgE was localized in the Fc region, which is known to carry the cytophilic site. Antigenic analysis showed that determinants localized in the Fc” region were unaffected by exposure to 56” for up to 150 min. However, determinants restricted to the two COOH-terminal domains of the E chain were progressively lost upon heating IgE over the same time period. The data indicate that the thermal sensitivity of IgE is restricted to that region which interacts with cell membranes and is not due to a general unfolding of the molecule.