n. [ Gr. ke`ras, -atos, horn. ] (Physiol. Chem.) 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. [ 1913 Webster +PJC ]

n. [ Neuro- + keratin. ] (Physiol. Chem.) A substance, resembling keratin, present in nerve tissue, as in the sheath of the axis cylinder of medullated nerve fibers. Like keratin it resists the action of most chemical agents, and by decomposition with sulphuric acid yields leucin and tyrosin. [ 1913 Webster ]