Ommochrome synthesi

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. " "... 

Hartnup s disease, a hereditary defect associated with mental retardation, is due to a deficiency of tryptophan oxygenase. The v -> v mutation in Drosophila melanogaster, manifested as defective ommochrome synthesis, is due to a deficiency of tryptophan oxygenase it can be reversed by the injection of kynurenine. 

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. ... 

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. 

Butenandt has similarly shown that the ommochrome eye pigments found in Drosophila species were not formed in some mutants imless kynurenine (24) or 3-hydroxykynurenine (88) was provided in the diet. In addition xanthommatin (84), rhodommatin and ommatin C, ommochromes synthesised by the blowfly Callio-phora erythrocephala, were demonstrated to possess radioactivity after injection of either d L-3-[ C]-tryptophan or D l-5-[ C]-kynurenine . Based on these observations Butenandt proposed that xanthommatin (84) was biosynthesised by an oxidative condensation of two molecules of 3-hydroxykynurenine (88) and a biogenetically patterned synthesis of (84) was achieved by treatment of 3-hydroxykynurenine (88) with potassium ferri-cyanide. The synthesis proceeds probably by oxidation to the... 

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). 

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