Ommochromes from tryptophan

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. 

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. 

Kynurenine Metaholism. Kynurenine may be metabolized in five ways acetylation to iV -acetylkynurenine,i decarboxylation to kynuramine, oxidation to 3-hydroxykynurenine, cyclization to a quinoline derivative, and cleavage to yield anthranilic acid." The oxidation, cyclization, and cleavage reactions are components of major pathways of tryptophan metabolism. Ommochrome is composed of a series of heterocyclic condensed ring systems that have been shown to be derived from tryptophan via kynurenine. The individual steps in the enzymatic formation of the pigments have not separated. ... 

Phenoxazines also include yellow-to-brown and brownish-red pigments found in invertebrate animals and some vertebrates, for example as shielding pigments of eyes. The most important pigments are ommochromes that arise from tryptophan via kynurenine and 3-hydroxykynurenine. An example is a yellow pigment xanthommatin (9-69) containing bound aspartic acid. 

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). 

Figure 8.21 The formation of ommochromes and ommins from tryptophan via kynurenine...

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. " "... 

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. 

Several interesting and important metabolites are derived from l-tryptophan (22) and these include the pyridine ring of nicotinic acid (and hence of the various nicotinanide co-enzymes), serotonin (5 -hydroxytryptamine) a powerful vasoconstrictor found in mammalian nervous tissue, and eye pigments (the ommochromes) of certain insects. From a comparative point of view a distinctive feature of the metabolism of L-tryptophan is the widespread use of a... 

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