Insect cuticular sclerotization: a review.

Quinones and amino acids are usually compartmentally separated in living systems, however there are several junctions in which they meet, react and influence. It occurs mainly in wounded, cut or crushed plant material during harvest, ensiling or disintegrating cells. Diffusing polyphenols are oxidized by polyphenol oxidases (PPOs) to quinonic compounds, which associate reversibly or irreversibly with amino acids and proteins. The reaction takes place with the free nucleophilic functional groups such as sulfhydryl, amine, amide, indole and imidazole substituents. It results in imine formation, in 1,4-Michael addition via nitrogen or sulphur and in Strecker degradation forming aldehydes. The formation and activity of quinone–amino acids conjugates influences the colour, taste, and aroma of foods. Physical and physiological phenomena such as browning of foods, discoloration of plants during processing, alteration of solubility and digestibility, formation of humic substances, germicidal activity, cytotoxicity and more occur when quinones from disintegrating cells meet amino acids. The mechanisms of toxicity and the pathways by which PCBs may be activated and act as a cancer initiator include oxidation to the corresponding quinones and reaction with amino acids or peptides. Sclerotization of insect cuticle is a biochemical process involving also the reaction between quinones and amino acid derivatives.

When quinones meet amino acids: chemical, physical and biological consequences.

The insect cuticle is an extracellular structure covering the total outer surface of the animal and providing protection against harmful influences from the environment. The mechanical properties of cuticles may vary considerably, and pronounced regional differences are generally observed. The properties may also change during development, and it can be assumed that the physical and chemical properties of all cuticular regions tend to be close to the optimal for proper physiological function during all developmental stages. Cuticular regions can be stabilized by the process of sclerotization, whereby o-diphenols are oxidatively incorporated into the material. Our current knowledge of the sclerotization process is reviewed, and it is suggested that the main features of the chemistry of sclerotization probably have been established, and that the major questions now remaining concern the precise regional and temporal control of the process.

Cuticular sclerotization in insects

Publisher Summary The integument, the outer layer of the insect comprising the epidermis and the cuticle, is primarily responsible for the great success of insects as terrestrial animals. With regard to its rigidity and hardness, the cuticle not only supports the various body parts but also provides protection to the insect by restricting water loss and preventing invasion by foreign organisms. Hardened cuticle does not permit increase in body size or changes in forms therefore, insects during their life cycle, shed their old cuticle several times, and synthesize new ones that are soft and flexible initially to allow for expansion, but subsequently harden to a tough exoskeleton by a process known as tanning or sclerotization. Essentially, sclerotization protects eggs, egg cases, larval cuticle, pupal and puparial cuticle, and adult cuticle of insects. This chapter describes the recent advances in insect cuticular sclerotization. The three major mechanisms of cuticular sclerotization include: (1) Quinone tanning mechanism, (2) β -sclerotization, and (3) Quinone methide sclerotization. The discovery of quinine isomerase and quinone methide isomerase has resulted in the unification of these three different mechanisms into a single interlinked pathway.

Unified Mechanism for Sclerotization of Insect Cuticle

Publisher Summary This chapter discusses the cuticular sclerotization and tanning. The cuticle covers the insect body as an effective barrier between the animal and its surroundings; it provides protection against desiccation, microorganisms, and predators, and as an exoskeleton it provides attachment sites for muscles. Cuticle can occur as relatively hard and stiff regions, the sclerites, separated by more flexible and pliable cuticular regions, the arthrodial membranes, which make the various forms of locomotion possible. Cuticular sclerotization is a chemical process by which certain regions of the insect cuticle are transformed irreversibly from a pliant material into a stiffer and harder structure, characterized by decreased deformability, decreased extractability of the matrix proteins, and increased resistance to enzymatic degradation. During sclerotization the color of the cuticle may change; some cuticles remain nearly colorless, and some become lighter or darker shades of brown or black. The term “tanning” is often used synonymously with sclerotization, but sometimes it is specifically used for the processes whereby brownish (tan) cuticles are formed.

D Ricketts

Papers

1,2-dehydro-N-beta-alanyldopamine as a new intermediate in insect cuticular sclerotization.