Keratin

A sulfur containing fibrous protein constituting the main structural protein of hard epidermal tissues such as horn hair feathers nails claws hoofs and the like It is an insoluble substance and unlike elastin is not dissolved even by gastric or pancreatic juice By decomposition with sulphuric acid it yields leucine and tyrosine plus various other acid stable amino acids The amino acid composition varies but it usually has a high percentage of cystine which stabilizes and insolubilizes the protein by forming intrachain linkages A softer form of keratin is present in the epidermis and whalebone Called also epidermose