Formation of biologically inactive polymers is responsible for the thermal inactivation of rat IgE.

The structural changes induced by heating rat IgE at 56 degrees C and relationship with loss of cytotropic activity were examinated in the present study. Circular dichroism spectrum of IgE heated at 56 degrees C showed irreversible changes in the peptide bond spectral regions: increase in beta-sheet structure, but no significant modifications in the aromatic side chain region. Thus, circular dichroism studies did not suggest important perturbations of the tertiary structure of the IgE molecule. Parallel studies with F(ab')2-epsilon fragment did not show significant alterations of either peptide bond or aromatic side chain spectral regions. Analysis of IgE heated at 56 degrees C by polyacrylamide gradient gel electrophoresis showed the presence of large amounts of polymeric material. Polymerization of IgE was found to increase with time of heating at 56 degrees C and to depend on protein concentration; polymerization was decreased at temperatures lower than 56 degrees C. A relationship between loss of cytotropic activity and the proportion of polymeric material in the heated IgE solutions was observed. Isolated polymeric molecules produced by heating showed considerable decrease in cytotropic activity whereas monomer isolated from heated IgE was found biologically active. The ability to form polymers is an intrinsic property of the carboxy-terminal domains C epsilon 3 and C epsilon 4, as the F(ab')2-epsilon fragment did not polymerize upon heating at 56 degrees C. A model of thermal inactivation of rat IgE is proposed in which aggregation of the carboxy-terminal domains of the epsilon-chain does not allow interaction of these domains with the monovalent IgE receptor of mast cells.