Human hair keratin responds to oxidative stress via reactive sulfur and supersulfides

Keratin is a central component of human hair proteins, which explicitly possesses many cysteine residues (Cys-SH). For a long time, these Cys-SH residues were believed to contribute to human hair strength by forming intra- and inter-molecular disulfide bond crosslinks. However, we detected that many polysulfide bonds (R-SS_(n)H or R-SS_(n)S-R') exist in keratin. Polysulfide is one of the reactive sulfur and supersulfides, similar to cysteine persulfide (Cys-SSH), that regulates oxidative stress and redox signaling. In the present study, we elucidated the distribution of polysulfide in human hair and the reaction of polysulfide to various oxidative stress, such as heat shock and ultraviolet radiation. The decrease of the polysulfides in hair leads to the loss of antioxidant activity. Additionally, we demonstrated the effect of sulfur supplementation on human hair strength and hair cuticle structure. All types of oxidative stresses decreased the polysulfide in human hair, and hair polysulfide positively correlated with human hair strength. Intriguingly, sulfur supplementation improved human hair strength and the structure of hair cuticles. In conclusion, polysulfide in human hair keratin contributes to hair strength and antioxidant activity against oxidative stresses such as ultraviolet radiation and maintains hair homeostasis.