Demethylation of methionine and keratin damage in human hair

Abstract

Growing human head hair contains a history of keratin and provides a unique model for studies of protein damage. Here, we examined mechanism of homocysteine (Hcy) accumulation and keratin damage in human hair. We found that the content of Hcy-keratin increased along the hair fiber, with levels 5–10-fold higher levels in older sections at the hair’s tip than in younger sections at hair’s base. The accumulation of Hcy led to a complete loss of keratin solubility in sodium dodecyl sulfate. The increase in Hcy-keratin was accompanied by a decrease in methionine-keratin. Levels of Hcy-keratin were correlated with hair copper and iron in older hair. These relationships were recapitulated in model experiments in vitro, in which Hcy generation from Met exhibited a similar dependence on copper or iron. Taken together, these findings suggest that Hcy-keratin accumulation is due to copper/iron-catalyzed demethylation of methionine residues and contributes to keratin damage in human hair.

Mutations in Homocysteine Metabolism Genes Increase Keratin N-Homocysteinylation and Damage in Mice

“…We found that both Hcy-keratin and S -Hcy-keratin were present both in mammal and bird pelages at levels ranging from 56 to 733 pmol/mg hair ( Table 1 ). The higher content of Hcy-keratin and lower content of S -Hcy-keratin at the tip of the horse tail hair than at its base are similar to those observed previously in the long human head hair [ 18 ].…”

“…Our previous work has shown that Hcy, bound by a disulfide or an amide bond, is a component of human hair keratin [ 18 ]. To determine whether the presence of Hcy in hair keratin is a universal feature conserved across the animal world, we analyzed Hcy content in pelages of mammals and birds.…”

“…In a growing human hair, Hcy-keratin levels increase along the hair shaft as a result of copper and iron-dependent demethylation of methionine residues of keratin [ 18 ]. To determine whether this process could contribute to the accumulation of Hcy-keratin in HHcy, we quantified copper and iron levels in hair as well as Met content of hair keratin in Cse −/− and Cse +/+ mice.…”

“…In hair, two mechanisms can account for the presence of Hcy linked by an amide bond to keratin: (i) N -homocysteinylation of keratin lysine residues by Hcy-thiolactone and (ii) copper- and iron-dependent demethylation of keratin methionine residues [ 18 ]. Although we have not directly shown that Hcy is attached to a lysine residue in keratin, we have eliminated the 2 nd possibility by showing that copper, iron, and keratin methionine levels are not affected by HHcy in the Cse −/− mice ( Table 5 ).…”

“…The Met HPLC assay [ 18 ] has been used with the following modification. Briefly, mouse pelage hair (~10 mg) were hydrolyzed in 1 ml Wheaton Gold Band ampoules with 6N HCl (100 μ l, 120°C, 1 h).…”