Cuticle tanning

During the past few years, a number of peptides have been isolated from insects and plants with very powerful hormonal or toxic effects (18, 19). Chapter 7 in this volume describes work on the effects of enzymes on cuticle tanning that are hormonal like in actions and have activity at low concentrations. 

In insecta large amounts of DA are required for cuticle tanning processes. From U. S. von Euler, 1963, Vol. 2. 

The solid cuticle of the arthropod is a complex extracellular product made of chitin (a polysaccharide) and tanned or fibrous proteins, and is often hard and... 

Cutlcular permeability In Insects Is correlated with a number of factors Including growth stage of the Insect, surface to volume ratios, the degree of tanning In the cuticle, conditioning temperatures, and cutlcular lipids 2 ). The cuticle of... 

Cuticle. The cuticle of adult nematodes is relatively impermeable and is composed primarily of a complex of several proteins. These proteins include collagens, fibroids, elastoids, and keratoids, possibly hardened by tanning with pol3rphenols or quinones. Sometimes lipide materials are also present (8, 10y 52). When such lipide materials are present on the adult cuticle, they probably take the form of hydroxy fatty acids or esters of fatty acids with monohydroxy alcohols. Such materials may form ring compounds. Regardless of the exact chemical nature, the lipides are unquestionably one of the chief barriers to permeability. These materials are commonly difficult to demon-... 

The presence of catechols and more complex, oxidizable polyphenols in nature is widespread, and their functions are not limited to chemical defense. However, biological control of their oxidation is usually a feature of their function, as it is (1) in melanin synthesis,3 (2) in immunologically mediated delayed-type hypersensitivity responses,4 (3) in the hardening or curing of arthropod secretions (for example, as in the surface attachment adhesives of the barnacle and in tanning of the cuticle in insects),5 as well as (4) in defensive mechanisms in higher plants, particularly in the unleashing of immediate necrotrophic responses.6... 

Cleavage of 3-hydroxykynurenine by kynureninase (step g, Fig. 25-11) forms 3-hydroxyanthranilate, which is opened under the action of another dioxygenase (step h) with eventual degradation to acetyl-CoA, as indicated. In insects the reactive 3-hydroxyanthranilate is utilized in "tanning" reactions, e.g., coupling to tyrosine residues to toughen insect cuticles and walls of cocoons.214... 

Kennaugh (8) reports changes in the quantity of j3-alanine in the cuticle of the American cockroach, Periplaneta americana (L.), during hardening and tanning, but it is not clear whether this represents free or combined /3-alanine. Data presented herein and additional unpublished material indicate that the /3-alanine is, indeed, a constituent of the cuticular protein. 

If the compound is causing the insect to be stressed or intoxicated, the insect larva (in this case a prepupa) may have trouble casting the old cuticle, yet be capable of tanning. Thus, the insect becomes a tanned larva or what appears to be a half pupa/half larva (due, in part, to incomplete ecdysis). For example, larvae treated with paraoxon (200 nmol./larva data not included in Table I due to high mortality) caused the formation of tanned larvae in all larvae treated. However, all of these larvae had obviously been adversely affected (intoxicated) by the paraoxon... 

Furthermore, many insects, particularly the adults, are protected against the entry of contact insecticides by thick and sclerotized cuticle (Ebeling, 1974). For example, newly molted American cockroach adults picked up three to four times more malathion than those that were darkly tanned (Matsumura, 1959). 

The crustacean cuticle is made of chitin and protein and has an outer tanned epicuticle within which lies a tanned and calcified exocuticle. The thickest layer is the inner endocuticle which is heavily calcified but contains no tanned problems. The inside of the exoskeleton is formed by a membranous layer which is neither tanned nor mineralized but which covers the cellular epidermis. This layer contains a variety of cell types including melanophores and epidermal cells but the secretion of the exoskeleton is mainly performed by the cuticle-secreting cells and the associated intra-epidermal or reserve cells. The exoskeleton is penetrated by tegumental ducts and by pore canals. The first of these carry secretions from glands through the thickness of the cuticle while the latter may consist of 50—90 fine protoplasmic extensions of each cuticle-secreting cell penetrating the exoskeleton at a density of about 4 X 10 pore canals mm". ... 

At present relatively little is known about the chemistry of the matrix protein, although significant differences have been reported between proteins extracted from different tissues. Extraction data indicate the presence of numerous protein moieties, (23, 24, 25, ) but interpretation is difficult because of extensive cross-linking (tanning) in mature cuticles. Rudall and Kenchington (21) report that the evidence for ordering of the protein is eliminated by extraction with 7M urea, suggesting a non-covalent interaction between protein and chltln. 

SO Anderson. Sclerotization and tanning of the cuticle. In Comprehensive Insect Physiology, Biochemistry and Pharmacology, Vol. 3. GA Kerkut, LI Gilbert, eds. New York Pergammon Press Ltd., 1985, pp. 59-74. 

When new cuticle Is synthesized, It Is soft and flexible so that the hydrostatic pressures unfold and expand the new cuticle thereby Increasing its surface area and concomitantly casting off the old cuticle. After ecdysis, expansion of the new cuticle Is brought to an end by the onset of sclerotlzatlon (tanning) (2), which Involves the cross-linking of cutlcular protein with ortho-qulnone The source of the ortho-qulnone Is tyrosine, whose mobilization Is controlled by eedysterone, as well as by a peptide hormone called bursIcon. 

Quinone causes tanning and hardening of the larval cuticle, which is a precondition of moulting. This process, known as sclerotisation, is a result of the reaction between the quinone and the cuticle proteins. One of the basic steps of quinone formation is the decarboxylation of DOPA into dopamine, effected by DOPA decarboxylase. Presumably, the messenger-RNA responsible for DOPA decarboxylase synthesis is formed by the action of ecdysone. 

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