Occurrence and characterization of a labile xanthommatin precursor in some invertebrates

Uncyclized xanthommatin is a key ommochrome intermediate in invertebrate coloration

Ommochromes are widespread pigments that mediate multiple functions in invertebrates. The two main families of ommochromes are ommatins and ommins, which both originate from the kynurenine pathway but differ in their backbone, thereby in their coloration and function. Despite its broad significance, how the structural diversity of ommochromes arises in vivo has remained an open question since their first description. In this study, we combined organic synthesis, analytical chemistry and organelle purification to address this issue. From a set of synthesized ommatins, we derived a fragmentation pattern that helped elucidating the structure of new ommochromes. We identified uncyclized xanthommatin as the elusive biological intermediate that links the kynurenine pathway to the ommatin pathway within ommochromasomes, the ommochrome-producing organelles. Due to its unique structure, we propose that uncyclized xanthommatin functions as a key branching metabolite in the biosynthesis and structural diversification of ommatins and ommins, from insects to cephalopods.

Xanthommatin is Behind the Antioxidant Activity of the Skin of Dosidicus gigas

Marine bioactive compounds have been found in very different sources and exert a very vast array of activities. Squid skin, normally considered a discard, is a source of bioactive compounds such as pigments. Recovering these compounds is a potential means of valorizing seafood byproducts. Until now, the structure and molecular properties of the bioactive pigments in jumbo squid skin (JSS) have not been established. In this study, methanol-HCl (1%) pigment extracts from JSS were fractionated by open column chromatography and grouped by thin-layer chromatography in order to isolate antioxidant pigments. Antioxidant activity was determined by the 2,2-diphenyl-1-picrylhydrazyl (DPPH^●) and 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^●+) radical scavenging assays and ferric reducing power (FRAP) assay. Fractions 11-34 were separated and grouped according to flow rate values (F1-F8). Fractions F1, F3, and F7 had the lowest IC₅₀ against ABTS^●+ per milligram, and fractions F3 and F7 showed the lowest IC₅₀ in the FRAP assay. Finally, fraction F7 had the highest DPPH^● scavenging activity. The chemical structure of the F7 fraction was characterized by infrared spectroscopy, ¹H nuclear magnetic resonance, and electrospray ionization-mass spectrometry. One of the compounds identified in the fraction was xanthommatin (11-(3-amino-3-carboxypropanoyl)-1-hydroxy-5-oxo-5H-pyrido[3,2-a]phenoxazine-3-carboxylic acid) and their derivatives (hydro- and dihydroxanthommatin). The results show that JSS pigments contain ommochrome molecules like xanthommatin, to which the antioxidant activity can be attributed.

Ommochromes in invertebrates: biochemistry and cell biology

Ommochromes are widely occurring coloured molecules of invertebrates, arising from tryptophan catabolism through the so-called Tryptophan → Ommochrome pathway. They are mainly known to mediate compound eye vision, as well as reversible and irreversible colour patterning. Ommochromes might also be involved in cell homeostasis by detoxifying free tryptophan and buffering oxidative stress. These biological functions are directly linked to their unique chromophore, the phenoxazine/phenothiazine system. The most recent reviews on ommochrome biochemistry were published more than 30 years ago, since when new results on the enzymes of the ommochrome pathway, on ommochrome photochemistry as well as on their antiradical capacities have been obtained. Ommochromasomes are the organelles where ommochromes are synthesised and stored. Hence, they play an important role in mediating ommochrome functions. Ommochromasomes are part of the lysosome-related organelles (LROs) family, which includes other pigmented organelles such as vertebrate melanosomes. Ommochromasomes are unique because they are the only LRO for which a recycling process during reversible colour change has been described. Herein, we provide an update on ommochrome biochemistry, photoreactivity and antiradical capacities to explain their diversity and behaviour both in vivo and in vitro. We also highlight new biochemical techniques, such as quantum chemistry, metabolomics and crystallography, which could lead to major advances in their chemical and functional characterisation. We then focus on ommochromasome structure and formation by drawing parallels with the well-characterised melanosomes of vertebrates. The biochemical, genetic, cellular and microscopic tools that have been applied to melanosomes should provide important information on the ommochromasome life cycle. We propose LRO-based models for ommochromasome biogenesis and recycling that could be tested in the future. Using the context of insect compound eyes, we finally emphasise the importance of an integrated approach in understanding the biological functions of ommochromes.

Xanthommatin-Based Electrochromic Displays Inspired by Nature

Color is a signature visual feature in nature; however, the ability to trigger color change in the presence of different environmental stimuli is unique to only a handful of species in the animal kingdom. We exploit the natural color-changing properties of the predominant pigment in arthropods and cephalopods-xanthommatin (Xa)-and describe its utility as a new broad-spectrum electrochromic material. To accomplish this goal, we explored the spectroelectrochemical properties of Xa adsorbed to an indium-doped tin oxide-coated substrate chemically modified with poly(3,4-ethylene dioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). We identified a synergistic role between PEDOT:PSS and Xa that contributed to its absorption profile, which could be modulated across multiple cycles. By varying the ratio of the two electroactive components, we also altered the perceived visible color of Xa-based devices, which cycled from different shades of red to yellow under reducing and oxidizing potentials, respectively. Together, our data illustrate the utility of Xa-based devices as new broad-spectrum electrochromic materials.