Ommochrome

Ommochrome is a class of pigments produced by invertebrates, especially within the pigment cells of the eye and, in some species, the body. They contribute to the distinctive coloration of the eyes in many insects, arachnids, and mollusks and can influence light filtering and signaling. Ommochromes originate from the tryptophan–kynurenine metabolic pathway and occur mainly as two structural families: ommatins and ommins.

Ommatins, including xanthommatin and dihydroxanthommatin, typically yield yellow to orange-red hues. Ommins are sulfur-containing pigments that tend to produce purple colors. Both classes arise from oxidative condensations involving 3-hydroxykynurenine and related intermediates within pigment granules in pigment cells.

Biosynthesis begins with tryptophan, which is converted to kynurenine and then to 3-hydroxykynurenine. This intermediate undergoes oxidative dimerization and cyclization to form xanthommatin (an ommatin), with further modifications yielding other ommatins. Sulfur-containing units can be incorporated to generate ommins, resulting in the purple end of the spectrum.

Distribution and function vary by species but commonly include the retinal epithelium or associated pigment cells in the eyes, where ommochromes help screen light and protect photoreceptor tissues. They may also contribute to body coloration in some invertebrates. Ommochromes are typically insoluble in water and are stored in specialized pigment granules.

In research and taxonomy, ommochrome patterns are used to study development, vision, and phylogenetic relationships among invertebrates. They represent a major, chemically diverse pigment system alongside other pigments such as pterins in many taxa.