Xanthurenic Acid in the Shell Purple Patterns of Crassostrea gigas: First Evidence of an Ommochrome Metabolite in a Mollusk Shell.

Michel Bonnard,Bruno Boury,Isabelle Parrot

doi:10.3390/molecules26237263

Abstract

Ommochromes are one of the least studied groups of natural pigments, frequently confused with melanin and, so far, exclusively found in invertebrates such as cephalopods and butterflies. In this study focused on the purple color of the shells of a mollusk, Crassostrea gigas, the first evidence of a metabolite of ommochromes, xanthurenic acid (XA), was obtained by liquid chromatography combined with mass spectrometry (UPLC-MS). In addition to XA and various porphyrins previously identified, a second group of high molecular weight acid-soluble pigments (HMASP) has been identified with physicochemical and structural characteristics similar to those of ommochromes. In addition, fragmentation of HMASP by tandem mass spectrometry (MS/MS) has revealed a substructure common to XA and ommochromes of the ommatin type. Furthermore, the presence of melanins was excluded by the absence of characteristic by-products among the oxidation residues of HMASP. Altogether, these results show that the purple color of the shells of Crassostrea gigas is a complex association of porphyrins and ommochromes of potentially ommatin or ommin type.

Highlights

Molluscan shell pigments are generally assigned to carotenoids, melanins and tetrapyrroles [1]
The precise chemical structure of ommochromes is generally unknown at present, their occurrence in a natural sample is usually postulated by the identification of specific biosynthetic metabolites such as 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HA) and xanthurenic acid (XA) [10,11,12,13]
Our previous study having established the presence of porphyrins [7], the present study focuses on the composition of porphyrins-free acid-soluble pigments of purple patterns of shells of C. gigas in order to establish the absence or presence of melanins and ommochromes, carotenoids were not considered, being the only group of acid-insoluble

Summary

Introduction

Molluscan shell pigments are generally assigned to carotenoids, melanins and tetrapyrroles [1]. Uroporphyrin and derivatives were identified in the mantle of C. gigas and the purple and dark patterns of its shell [7], constituting an evidence of the hemebased cellular respiration of C. gigas [8] These represent only a small proportion of the overall acid-soluble pigments among which, the occurrence of ommochromes would corroborate the recent identification of genes associated with their biosynthetic pathways [9]. Our previous study having established the presence of porphyrins [7], the present study focuses on the composition of porphyrins-free acid-soluble pigments of purple patterns of shells of C. gigas in order to establish the absence or presence of melanins and ommochromes, carotenoids were not considered, being the only group of acid-insoluble. CA.fgfeignazseallsearnetaassl.o[c5ia].tiAonltoogf eatchider-s, othluisblleeapdosrpuhsytroincsonansiddear thtyeppeuorfpolme mcoolochrroofmCe.s.gigas as an association of acid-soluble porphyrins and a type of ommochromes

Results

Comparative Analysis with Natural Melanin

Characterization of PF

Comparative Analysis with Natural Eumelanin

Chemicals