| Summary: | The harpacticoid copepod <i>Tigriopus californicus</i> has been recognized as a model organism for the study of marine pollutants. Furthermore, the nutritional profile of this copepod is of interest to the aquafeed industry. Part of this interest lies in the fact that <i>Tigriopus</i> produces astaxanthin, an essential carotenoid in salmonid aquaculture. Here, we study for the first time the stereochemistry of the astaxanthin produced by this copepod. We cultured <i>T. californicus</i> with different feeding sources and used chiral high-performance liquid chromatography with diode array detection (HPLC-DAD) to determine that <i>T. californicus</i> synthesizes pure 3<i>S</i>,3’<i>S</i>-astaxanthin. Using <i>meso</i>-zeaxanthin as feed, we found that the putative ketolase enzyme from <i>T. californicus</i> can work with β-rings with either 3<i>R</i>- or 3<i>S</i>-oriented hydroxyl groups. Despite this ability, experiments in the presence of hydroxylated and non-hydroxylated carotenoids suggest that <i>T. californicus</i> prefers to use the latter to produce 3<i>S</i>,3’<i>S</i>-astaxanthin. We suggest that the biochemical tools described in this work can be used to study the mechanistic aspects of the recently identified avian ketolase.
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