Keratin, a simple protein of albuminoid nature, is the chief constituent of hair, indeed of all epidermal tissue—nails, feathers, horns and hoofs. Keratin is peculiar, in that it has a high sulphur content, the sulphur being present almost entirely in the form of the amino-acid cystine. No other protein is so high in cystine as the keratin of human hair. It would appear, therefore, that the metabolism of sulphur probably plays an important rôle in the development and growth of these tissues.
SULPHUR AND CYSTINE CONTENT OF KERATIN; RELATION OF CYSTINE TO GROWTH OF HAIR
Vauquelin,1 in 1806, was apparently the first to report the chemical composition of hair. He found that hair contained considerable sulphur, and that white, blond and red hair contained more sulphur than black hair. Later, van Laer1 found that the sulphur content of hair of men varied between 4.6 and 5.4
BROWN H, KLAUDER JV. SULPHUR CONTENT OF HAIR AND OF NAILS IN ABNORMAL STATESTHERAPEUTIC VALUE OF HYDROLYZED WOOL; I. HAIR. Arch Derm Syphilol. 1933;27(4):584–604. doi:10.1001/archderm.1933.01450040591004