Sklerotine (Greek σκληρός skleros "hard" ) are by quinone derivatives solidified structural proteins, which are in the exoskeleton of arthropods ( arthropods such as insects and crustaceans ), an essential component. They come out in animals ( arthropods and flatworms) also occurs in protists. The incorporation of sclerotin causes a hardening of anatomical structures, especially on mechanical stress points.
In the cuticle of insects sclerotin occurs at a proportion of 50-70 % as a rule together with chitin. Chitin without sclerotin allows flexibility of these areas. In the exoskeleton of marine crustaceans its share is much lower here the incorporation of calcium salts that by calcification of the required strength.
In flatworms occurs in the egg shells of trematodes and Turbellaria, is of the yolk Stock cells ( Vitellocyten ) of the germ yolk Stocks synthesized. In protists it is a main component of the protective outer layer of the oocysts of coccidia.
In sclerotization phenol and quinone compounds are covalently bonded to aminoreiche structural proteins by the enzyme phenoloxidase ( as polyphenol oxidase EC 18.104.22.168 ). Sclerotisation effected by a polymerization of the components, the matrix thus formed is rigid, it can not be enzymatically degraded, and embeds the additional components of the cuticle, a fixing. The sclerotisation includes covalent bonds with amino groups of quinones, of chitin and amino acids of other proteins and results in a brown or black color of the Arthropodins, for example in the cuticle of beetles. In conjunction with lipids sclerotin is responsible for the water-repellent properties of the insect cuticle.