Pigments of insects

Neurotoxins produced by the body. Some normal body constituents are neurotoxic in excess. These incluse quinolinic acid (Fig. 25-11),889 3-hydroxykynurenine (Fig. 25-11 p. 1444),890 and homocysteine.891 Elevated levels of homocysteine are also associated with vascular disease and stroke (Chapter 24). 3-Hydroxykynurenine is a precursor to ommochrome pigments of insects and an intermediate in conversion of tryptophan into the nicotinamide ring of NAD in humans (Fig. 25-11). 6-Hydroxydopamine (Fig. 30-26), which may be formed in the body, is severely toxic to catecholaminergic neurons.892... 

Tetrahydrobiopterin and tetrahydrofolic acid are coenzymes of monooxygenases (C 2.6) and of enzymes of the Ci-metabolism, respectively (D 3.2). Folic acid is a vitamin for humans, being reduced in the human body to tetrahydrofolic acid (E 2.1). Pterins, e.g., xanthopterin and leucopterin, sepiapterin and isosepiapterin are wing pigments of insects, for instance, of butterflies and flies. Similar compounds are skin pigments of fishes, amphibians, and reptiles and in this respect are of ecological significance. 

Botanists have been interested in the plant hormone heteroauxin which is indoleacetic acid. It occurs in the urine and has been shown to be a tryptophan metabolite in plants (see 7J). Another focus of general biological interest of tryptophan is that it is the precursor of the brown eye pigments of insects, ommochrome (875). 

Initially, 3-hydroxykynurenine was not discovered as the intermediate for nicotinic acid formation but rather for ommochrome formation (Butenandt and co-workers). Ommochromes are pigments found chiefly among insects and crabs. The simplest representative, xarUhommaline (formula in Chapt. VII-6) easy to prepare in vitro by careful oxidation of hydroxykynurenine. The synthesis of ommochromes is disrupted in several mutants of the fruitfly Drosophila (and other insects) either the transition from tryptophan to kynurenine or its oxidation to hydroxykynurenine is blocked. The pigmentation of insect eyes was one of the first examples of the thesis that hereditary factors control biochemical reactions (cf. Chapt. VII-6). Other examples, even in man, are provided by the metabolism of tyrosine (cf. Chapt. VII-6). 

Phenoxazines — The microbial phenoxazines like actinomycins are well-known antibiotics. Actinomycin D produced by Streptomyces anibioticus is an effective antineoplastic agent that inhibits nucleic acid synthesis. The main function of ommochromes is to act as screening pigments in the eyes of insects and other arthropods, as pattern pigments in the integument, and as excretion products of excess tryptophan. ... 

To extract and evalnate the color pigments from cochineals Dactylopius coccus Costa), a simple method was developed. The procednre is based on the solvent extraction of insect samples nsing methanol and water (65 35, v/v) and a two-level factorial design to optimize the solvent extraction parameters temperature, time, methanol concentration in mixtnre, and yield. For hydrophilic colorants that are more sensitive to temperatnre, water is the solvent of choice. For example, de-aerated water extraction at low temperatnre was applied to separate yellow saffrole and carthamine from saffron (Carthamus tinctorius) florets that contain about 1% yellow saffrole and 0.3% red carthamine. ... 

Chalcones (l,3-diaryl-2-propen-l-ones) are open-chain fiavonoids that are widely biosynthesized in plants. They are important for the pigmentation of flowers and, hence act as attractants to the pollinators. As fiavonoids, chalcones also play an important role in defending against pathogens and insects. A review of the literature concerning the evaluation of natural chalcones from medicinal plants reveals that many studies into their antimicrobial and antiviral activities have been carried out in recent years. Some of these compounds were isolated by bioassay-guided fractionation, after previously detecting activity on the part of the plants. 

The remarkable matching of insect pigmentation to that of the host plant led to the idea that the pigment of host and pest were related, possibly identical. Early workers compared the green... 

Recent investigations have been focused on the identification of the protein-pigment complexes of insects. For example, in the tobacco hornworm, Manduca sexta, a blue biliprotein, insecticyanin, has been found in the hemolymph, epidermal cells and eggs (6). 

Pigment-Dispersing Actions of Insect Nervous Tissue Extracts on Crustacean Chromatophores... 

Sequence Similarities of Insect Pigment-Dispersing Factors and Crustacean Pigment-Dispersing Hormones... 

Tryptophan is probably the indole derivative most widely distributed in nature. It is converted into many other substances of important biological significance. The many materials biogenetically related to tryptophan include nicotinic acid (a vitamin), serotonin (a neurohormone), indoleacetic acid (a phytohormone), some pigments found in the eyes of insects, and a number of alkaloids. 

Another aspect, through which the degradation of tryptophan becomes a part of the chapter of the genes, must be remembered for its great biological importance. In 1952 Butenandt (B29) succeeded in clarifying the structure of ommochromes. These pigments of the eye of insects, which in the past were considered as pterin derivatives, are today known to be formed directly from kynurenine and 3-hydroxykynurenine. 

Kynurenine and 3-hydroxykynurenine are the precursors or chromogens of the pigments. One of them, xanthommatin, was obtained from the eyes of insects 7800 heads of Calliphora erythrocephala gave 19 mg of the pigment (B32). Xanthommatin was also isolated from the molting secretion of Vanessa urticae 10,000 butterflies yielded 100 mg of the substance (B30). 

The major pigment of cochineal is polyhydroxyanthraquinone C-glycoside, car-minic acid (Formula 9.16), which may be present at up to 20% dry weight of the mature insects. Cochineal extract or carminic acid are rarely used as coloring materials for food, but are usually offered in the form of their lake. Aluminum complexes (lakes) can be prepared with ratios of cochineal and aluminum varying from 8 1 to 2 1, having corresponding shades from pale yellow to violet. 

The identification of hydroxykynurenine as the cn+-substance which is the precursor of insect eye pigments (ommochromes) has already been described. Ommochromes had previously been considered to be pterin derivatives (140, 534), but there is no obvious route for the conversion of hydroxykynurenine to the pterin type of structure. However, pterins also occur in insect eyes (275). More recent work has shown that the eye pig-... 

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