The metabolism of the aromatic amino acids concerned in the cross-linking of insect cuticle

Insects lay eggs that must survive in nature often several days to months. The eggs need to be protected from desiccation and other harsh environmental challenges. The structural components of eggshells are mostly proteins, but in some cases the polysaccharide, chitin is also used as an essential matrix material. In addition, several eggshells also contain minerals and catecholamine derivatives. Enzymic components often include peroxidase, laccase and/or phenoloxidase and other sclerotizing enzymes. In this review, some of the aspects of eggshell and chorion tanning in insects are reviewed. Four major systems representing different way of hardening are considered. They are hardening of the cockroach ootheca, tanning of the mosquito egg chorion, crosslinking of Drosophila eggshell, and sclerotization of praying mantis egg sacs. Cockroach ootheca, which is devoid of chitin, mostly use sclerotization mechanism to harden its case using 3,4-dihydroxybenzyl alcohol. Mosquito eggs which possess chitin on the other hand, employ both melanisation and peroxidase mediated dityrosine and trityrosine formation for tanning reactions. Drosophila egg chorion primarily utilize peroxidase mediated crosslinking process only. While these three systems seem to differ from the normal sclerotization process employed by most insect cuticles which use N-acetyldopamine and N-β-alanyldopamine as sclerotizing precursors and phenoloxidase (both laccase and o-diphenoloxidase), quinone isomerase, and quinone methide isomerase as the sclerotizing enzymes, the egg sacs of praying mantis recruit a variety of N-acyldopamine derivatives and the two enzymes—phenoloxidase and quinone isomerase—for hardening their sacs. In addition, tanning of silk is also considered briefly in this review. The biochemical mechanisms of these reactions are discussed in detail.