Chemistry of insect cuticle. 3. Hardening and darkening of the cuticle

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Abstract
For an insect cuticular protein to become tanned in the presence of the enzyme polyphenol oxidase, the presence of an added o-dihydric phenol is not essential. It is concluded that the phenolic groups of the tyrosine residues in the protein become oxidized to an o-quinone. This quinone then acts as the tanning agent. Possible roles played by glutamic acid and tyrosine in this tanning (hardening) process are considered in some detail. The mechanism of the tanning reaction and the origin of the o-dihydric phenols is discussed. The tanned protein (sclerotin) is considered on spectroscopic evidence to be quite different from melanin. Spectroscopy is the only method known which can be used to distinguish between these two types of pigment. Sclerotin indicates a group of pigments formed by reaction between a protein and a quinone, whereas melanin is an ill-defined term which has come to include, besides the pigment formed by the enzymic oxidation of tyrosine,almost any naturally occurring pigment which is dark in color.
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