Investigation of the physiochemical properties, cryoprotective activity and possible action mechanisms of sericin peptides derived from membrane separation

In order to exploit their industrial applications, sericin peptides (3K-SP) were obtained by membrane separation and their physiochemical properties and cryoprotective function were investigated. Results showed that 3K-SP were mostly distributed less than 3000 Da, and rich in the amino acids Ser, Asp, Gly, Thr and Glu, which have been associated with the cryoprotective activity of ice-structuring proteins. Addition of 3K-SP to a frozen solution led to the reductions in melting temperature and melting time compared to control solution. In addition, 3K-SP inhibited ice recrystallization, since it could maintain small ice crystal sizes within a frozen solution. Furthermore, 3K-SP demonstrated high cryogenic protection activity of Lactobacillus delbrueckii Subsp. Bulgaricus during freezing, and provided optimal protection of cells at conditions in which its concentration was 1.0 mg/mL and the pH of the solution was 7.0. In these conditions, the percentage of surviving cells was as high as 84.78 ± 3.07%. Flow cytometric and scanning electron microscopy analyses showed that treatment with 3K-SP increased the percentage of viable cells from 52.9% to 80.1%, and suggest that 3K-SP may mediate its protective effects through interaction with cell membranes, whereby it surrounds cells in a glassy matrix that helps maintain their membrane integrity.