Critical Analysis of the Melanogenic Pathway in Insects and Higher Animals

Abstract

Animals synthesize melanin pigments for the coloration of their skin and use it for their protection from harmful solar radiation. Insects use melanins even more ingeniously than mammals and employ them for exoskeletal pigmentation, cuticular hardening, wound healing and innate immune responses. In this review, we discuss the biochemistry of melanogenesis process occurring in higher animals and insects. A special attention is given to number of aspects that are not previously brought to light: (1) the molecular mechanism of dopachrome conversion that leads to the production of two different dihydroxyindoles; (2) the role of catecholamine derivatives other than dopa in melanin production in animals; (3) the critical parts played by various biosynthetic enzymes associated with insect melanogenesis; and (4) the presence of a number of important gaps in both melanogenic and sclerotinogenic pathways. Additionally, importance of the melanogenic process in insect physiology especially in the sclerotization of their exoskeleton, wound healing reactions and innate immune responses is highlighted. The comparative biochemistry of melanization with sclerotization is also discussed.

Suppression of Laccase 2 severely impairs cuticle tanning and pathogen resistance during the pupal metamorphosis of Anopheles sinensis (Diptera: Culicidae)

“…The metabolism of melanin with the hydroxylation of tyrosine to dihydroxy phenylalanine (dopa) by the rate-limiting enzyme tyrosine hydroxylase (TH), followed by the decarboxylation of dopa to dopamine by the dopa decarboxylase (DDC) [5, 17–19]. These melanin precursors must be further oxidized by phenol oxidases (POs) to quinones and quinone methides [19–25].…”

“…These melanin precursors must be further oxidized by phenol oxidases (POs) to quinones and quinone methides [19–25]. Subsequently, the quinones and quinone methides conjugate with cuticular proteins to construct and tan the cuticle, and are also involved in melanotic encapsulation [3, 5, 14, 15, 17, 18, 22–25]. Laccase, a member of the phenol oxidase family, conservatively contains three cupredoxin-like domains and four copper ions that reside in a T1 copper site and a T2/T3 tricopper center [5, 26].…”

“…Subsequently, the quinones and quinone methides conjugate with cuticular proteins to construct and tan the cuticle, and are also involved in melanotic encapsulation [3, 5, 14, 15, 17, 18, 22–25]. Laccase, a member of the phenol oxidase family, conservatively contains three cupredoxin-like domains and four copper ions that reside in a T1 copper site and a T2/T3 tricopper center [5, 26]. Laccases oxidize a broad range of substrates, including polyphenol, methoxy-substituted phenol, aminophenol, and phenylenediamine [5, 27–29].…”

Anti-melanogenic Activity of Auraptene via ERK-mediated MITF Downregulation

“…It plays an important protective role against UV light-induced skin damage [4]. Melanin is secreted by melanocyte cells in the basal layer of the epidermis [5]. The melanocytes may produce excessive melanin (melasma) or result in other hyperpigmentation diseases under chronic sun exposure [6,7].…”

Introductory Chapter: Melanin, a Versatile Guardian

“…In mollusks other than cephalopods, melanin, together with other chromophores, serves to produce color paterns in their shells [4]. In insects, melanin is used even more resourcefully, not only for pigmentation of the exoskeleton but also for cuticle hardening, wound healing and in their innate immune responses [5]. Melanin in birds is responsible for coloration of the plumage and the exposed bare parts of the integument [6].…”