Ommochromes and Ommins

This discussion of pigments does not exhaust the list of coloured substances from insects. Some mention should be made of the ommochrome pigments from insect eyes. They function as screening pigments to cut out stray light. They can also be found in insect integument. They are divided into the ommatins of lower molecular mass, labile to alkali, and ommins, of higher molecular mass and stable to alkali. They are extracted from the ommatidia of the compound eyes. The ommatins are formed from the amino-acid tryptophan via kynurenine and hydroxykynurenine, which undergoes oxidative dimerization to give the ommatins. Yellow xanthommatin (the most frequently encountered,

^ O ^^ O-glucose rhodommatin ommin A

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

^ O ^^ O-glucose rhodommatin ommin A

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

Figure 8.21) and red dihydroxanthommatin are examples of ommochromes, derived from tyrosine. Use of labelled xanthommatin has shown it is converted to rhodommatin. Much has been learned about ommatins from studies of the fruit fly Drosophila melanogaster and its many eye-colour mutants. There are about 10 to 15 of these pigments in insects, from brown-yellow through deep red to purple. Loss of the amino-acid chain from hydroxykynurenine and dimerization gives cin-nabarinic acid, a compound found only in the commercial silk moth Bombyx mori.

Ommins are polymeric, insoluble and some contain sulphur. They are less well investigated. The structure of the trimer ommin A (Figure 8.21) suggests that they may all be based on linear polymers of hydroxykynurenine. Using 35S-labelling, it has been shown that the sulphur in ommins can be derived from cysteine or methionine, but not from sulphate, thiocyanate or sulphide.

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