Secondary structural changes of homologous proteins, lysozyme and α-lactalbumin, in thermal denaturation up to 130 °C and sodium dodecyl sulfate (SDS) effects on these changes: comparison of thermal stabilities of SDS-induced helical structures in these proteins.

The thermal stability of two homologous proteins, lysozyme and α-lactalbumin, was examined by circular dichroism. The present study clearly showed two different aspects between the homologous proteins: (1) the original helices of lysozyme and α-lactalbumin were unchanged at heat treatments up to 60 and 40 °C, respectively, indicating a higher thermal stability of lysozyme, and (2) upon cooling to 25 °C, the original helices of lysozyme were never reformed after they were once disrupted, while those of α-lactalbumin, disrupted at a particular temperature range between 40 and 60 °C, were completely reformed. In addition, the structural changes were also examined in the coexistence of sodium dodecyl sulfate (SDS), which induced the formation of helical structures in these proteins at 25 °C. A distinct difference appeared in the thermal stabilities of the SDS-induced helices. All of the SDS-induced helices of lysozyme were disrupted below 60 °C, while those of α-lactalbumin at 10 mM SDS were unchanged up to 130 °C. A similarity was also fixed. Not only the SDS-induced helices but also the original helices of the two proteins were reformed upon cooling to 25 °C after the thermal denaturation below 100 °C in the coexistence of 10 mM SDS.