Protein solubility, secondary structure and microstructure changes in two types of undenatured type II collagen under different gastrointestinal digestion conditions

Abstract Sodium dodecyl sulfonate polyacrylamide gel electrophoresis (SDS-PAGE), circular dichroism spectroscopy (CD), transmission electron microscope (TEM) and atomic force microscope (AFM) were used to analyze the structural properties and gastrointestinal digestive characteristics of natural insoluble undenatured type Ⅱ collagen (IC Ⅱ) and pepsin soluble undenatured type Ⅱ collagen (SC Ⅱ). Results showed that, after gastric digestion, especially under low pH conditions, some thick and short collagen fibers were dissolved from IC Ⅱ, which was accompanied by the release of soluble protein with triple helix structure. As to SC Ⅱ, when gastric digestion was performed under higher pH conditions, collagen fibers were mildly thinned and curved, and the triple helix structure was slightly destroyed. However, those changes hardly occurred during the intestinal digestion. Moreover, the undenatured type Ⅱ collagen existing in SC Ⅱ digestive supernatant was 1.2-12.4 times higher than that of IC Ⅱ depending on the pH conditions. These results suggested that, it is more likely for SC Ⅱ to exert its activity of relieving arthritis, and it should be taken when the pH of gastric environment is high.