Effect of γ-cyclodextrin as a lyoprotectant for freeze-dried actinidin

Actinidin (ATD) is a cysteine protease found in kiwifruit. It is used to tenderize meat and to enhance the digestion of proteins in the small intestine. However, ATD is unstable during freeze-drying, which alters its bioactivity. It is well known that sugars have the ability to protect proteins from the stress of freeze-drying. In this study, we investigated the protective effect of various saccharides on the stability of ATD during freeze-drying. The ATD activities of the samples containing γ-cyclodextrin (CyD) showed only a small decrease, and compared with trehalose and sucrose, γ-CyD was a more effective stabilizer for ATD. Secondary structural changes in freeze-dried ATD were observed by circular dichroism spectroscopy and compared with the changes in stabilized samples. There was a close relationship between the α-helix content and the stabilization. The sugars stabilized the protein by suppressing the changes in the α-helix. Fourier transform infrared spectroscopy measurement showed that the amide I band of ATD with γ-CyD was shifted to a lower wavenumber compared with other sugars. Therefore, stronger hydrogen bonds may be formed between ATD and γ-CyD than between ATD and other sugars. The suppression of changes in the protein secondary structure accompanying the formation of hydrogen bonding between the protein and the sugar also contributed to the protective effect of the sugars.