Ommochromes

Phenoxazines — The two main types of phenoxazines are the ommochromes and the microbial phenoxazines. The biosynthesis of ommochromes occurs via the kynurenine pathway. The tryptophan amino acid is converted to formylkynurenine and then to kynurenine and 3-hydroxykynurenine. Not all the steps of ommochrome synthesis are completely elucidated yet. Ommatins are dimers and ommins are oligomers of 3-hydroxykynurenine. - The papiliochromes are derived from tyrosine as well as from the tryptophan pathway. The key intermediate in the formation of papiliochromes is N-beta-alanyldopamine (NBAD). Papiliochromes are synthesized in special wing scale cells, before melanins. " "... 

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. ... 

Martel, R.R. and Law, J.H., Purification and properties of an ommochrome-binding protein from the hemolymph of the tobacco homworm, Manduca sexta, J. Biol. Chem., 266, 21392, 1991. 

Ommochromes belong to the naturally occurring pigments present in many members of the animal kingdom mainly in insects. Their presence in the central nervous system of the silkworm Bombyx mori was proved by using cellulose and silica stationary phases. The Rp values of standards and the isolated pigments are compiled in Tables 2.107 and 2.108. 

H. Sawada, M. Nakagoshi, K. Mase and T. Yamamoto, Occurrence of ommochrome-containg pigment granules in the central nervous system of the silkworm Bombyx mori. Comp. Biochem. Physiol. Part B 125 (2000) 421 —428. 

The end products of tryptophan metabolism in the stick insect, Carausius morosus, are the ommochromes ommin and xanthommatin (58) in the epidermis, and kynurenic acid (53) in the feces. During larval development of this insect kynurenic acid (53) is the major end product of tryptophan metabolism (Table V) (109,110). Additionally, this insect contains five pteridines (Table VI), of which leucopterin (68), xanthopterin (65), and isoxanthopterin (67) are the origin of the yellow-white color of the insect (111). 

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... 

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. 

Ommochromes are acid dyes of red, yellow-brown, and purple color, practically insoluble in all neutral solvents. The biogenesis of ommochromes from tryptophan is influenced by inheritance factors which act at definite steps. Kynurenine is formed under the control of v+ genes, and from it 3-hydroxykynurenine is formed under control of cn+ genes. 

B31. Butenandt, A., Schiedt, U., and Biekert, E., fiber Ommochrome, II Alkalis-cher und fermentativer Abbau von Xanthommatin und Rhodommatin. Alkalischer Abbau der Kynureninseitenkette. Ann. 586, 229-239 (1954). 

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-... 

Hydroxytryptophan was not metabolized by a tryptophan-adapted strain of Pseudomonas (217) and was not attacked by the tryptophan peroxidase-oxidase system (217, 884). The enteramine and kynurenine pathways are quite distinct, as is supported by the facts that synthetic 5-hydroxykynurenine (124, 574), the expected product of tryptophan peroxidase-oxidase action, does not act as an ommochrome precursor in insects or as a nicotinic acid precursor in Neurospora (124). 

In insects, kynureninase is much less important than in mammals, so nearly no HA arises from tryptophan metabolism, but rather HK which is (auto)oxidised to ommochromes. The ommochrome pathway could be a defensive strategy against tryptophan toxicity, developed by organisms that have lost the ability to completely degrade this aminoacid. 

It has been hypothesised [239] that eumelanins derive biosynthetically from tyrosine, whereas pheomelanins of hair involve the participation of other monomers besides tyrosine, for example HK which can polymerise to a yellow pigment (an ommochrome-like substance ), owing to interferences with the main biosynthetic pathway of eumelanin synthesis. 

The phenoxazine moiety occurs in pigments of fungi, lichen and butterflies, and also in the ommochromes which are responsible for the colour of the pigments in some insects. Xanthommatin 36, which contains a quinonoid phenoxazone chromophore isolated from the secretion of the fox-moth, serves as an example ... 

Butenandt, A. Uber die Bildung der Insekten-Ommochrome. Forsch, u. 

C, Hi2N204, Mr 236.23 occurs as an intermediate in the oxidative degradation of L- tryptophan in many organisms and plays a role in the biosynthesis of nicotinamide and the "ommochromes. 

Ommochromes. A group of acidic, yellow and red phenoxazine pigments occurring in the eyes, wings, hatching secretions, and skin of arthropods, especially... 

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