**4. Conclusions**

In the present study, RSM was used to optimize the extraction process of PSC from *Nibea japonica* skin. The pepsin concentration of 1389 U/g, solid-liquid ratio of 1:57 and hydrolysis time of 8.67 h was found to be optimal for PSC extraction; giving a yield of 84.85%. The extracted PSC was then characterized as type I collagen using SDS-PAGE electrophoresis, and the FTIR analysis also revealed that PSC maintains its triple-helical structure. All PSCs were soluble in the pH range of 1.0–4.0 and decreases in solubility were observed at neutral or alkaline conditions. All PSCs had a decrease in solubility in the presence of sodium chloride, especially with a concentration above 2%. Further study will be performed to investigate whether this collagen can be used in biomedical applications and other fields.

**Acknowledgments:** This work was financially supported by the Natural Science Foundation of Zhejiang Province (grant No. LQ16H300001 and No. LQ15C200009), the National Natural Science Foundation of China (grant No. 81773629 and No. 21502170), the National Spark Program of China (grant No. 2015GA700044), the National Undergraduate Training Program for Innovation and Entrepreneurship (grant No. 201710340011), the Foundation of Zhejiang Educational Committee (grant No. Y201534400), the Public Welfare Program of Zhoushan (grant No. 2015C31012 and No.2016C41003) and the Zhejiang Xinmiao Talents Program (grant No. 2017R411009).

**Author Contributions:** Yunping Tang conceived and designed the experiments. Fangmiao Yu, Chuhong Zong, Shujie Jin, Jiawen Zheng performed the experiments. Nan Chen, Ju Huang, Yan Chen, Fangfang Huang and Zuisu Yang and Guofang Ding carried out statistical analysis of the date. All authors contributed to the writing of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
