**4. Conclusions**

Fucoidan was extracted from *S. japonica*, and further degraded into low-molecular-weight fucoidan by oxidant degradation. The average molecular weights of fucoidan and low-molecular-weight fucoidan were 136 and 9.5 kDa, respectively. The in vivo animal experiment revealed that fucoidan and low-molecular-weight fucoidan had protective effect on adriamycin-induced nephrotic syndrome. Fucoidan and LMWF treatment could significantly inhibit adriamycin-induced proteinuria and decrease the elevated BUN, SCr, TG, and TC level of rats in a dose-dependent manner. LMWF at the same dose had a better protective effect than fucoidan. Since fucoidan and LMWF had similar sugar constituents but different molecular weight, the difference in renoprotection between fucoidan and LMWF was mainly attributed to their discrepancy in molecular weight. Their renoprotective effect maybe partly related to their antioxidant effect.

**Author Contributions:** Conceptualization, Q.Z. and J.T.; methodology, J.T.; validation, J.W and N.W.; formal analysis, J.T. and L.G.; investigation, J.T. and Y.Y.; resources, Q.Z. ; data curation, Q.Z. and J.W.; writing—original draft preparation, J.T.; writing—review and editing, Q.Z.; project administration, Q.Z.; funding acquisition, Q.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Key R&D Plan of China (Grant No. 2018YFD0901104), Major Scientific and Engineering Projects of Innovation in Shandong Province (Grant No.2019JZZY010818), STS Program of Chinese Academy of Sciences (KFJ-STS-QYZD-195) and K.C. Wong Education Foundation, CAS.

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