Thermostability of Refolded Ovalbumin and S-Ovalbumin

Ovalbumin, a member of the serpin superfamily, is transformed into a thermostabilized form, S-ovalbumin, during storage of shell eggs or by an alkaline treatment of the isolated protein (ΔTm =8 °C). As structural characteristics of S-ovalbumin, three serine residues (Ser164, Ser236 and Ser320) take the D-amino acid residue configuration, while the conformational change from non-thermostabilized native ovalbumin is very small (Yamasaki, M., Takahashi, N., and Hirose, M., J. Biol. Chem., 278, 35524–35530 (2003)). To assess the role of the structural characteristics on protein thermostabilization, ovalbumin and S-ovalbumin were denatured to eliminate the conformational modulation effects and then refolded. The denatured ovalbumin and S-ovalbumin were correctly refolded into the original non-denatured forms with the corresponding differential thermostability. There was essentially no difference in the disulfide structures of the native and refolded forms of ovalbumin and S-ovalbumin. These data are consistent with the view that the configuration inversion, which is the only chemical modification directly detected in S-ovalbumin so far, plays a central role in ovalbumin thermostabilization. The rate of refolding of S-ovalbumin was greater than that of ovalbumin, indicating the participation, at least in part, of an increased folding rate for thermodynamic stabilization.