Production of melatonin and its isomers by yeasts and effects of tryptophan supplementation

It has been suggested that melatonin may represent a bioactive component in Mediterranean diet, because of its occurrence in some traditional plant foods and beverages. In particular, after the first discovery in grapes, this indoleamine and its isomers have been recently detected in many grapevine products (red, white and sweet wines, grape juice and vinegar). In order to ascertain a possible role of yeasts as melatonin-producers during winemaking, different species and strains were investigated in laboratory conditions. Saccharomyces cerevisiae EC1118, IOC18-2007 and UMY255, Torulaspora delbrueckii CBS1146, UMY196 and UMY336, and Zygosaccharomyces bailii ATCC36947 were inoculated in YNB medium (at approximately 105 Colony Forming Units/mL) supplemented with 2% (w/v) glucose and grown at 25°C. To promote melatonin production, parallel growth tests were carried out increasing the final concentration of its precursor, tryptophan, to 100 mg/L. Melatonin and its isomers were monitored every 24h till yeast cells reached the stationary phase of growth. Both supernatant and cellular content were submitted to the UPLC-MS/MS analysis. All the assayed yeasts did not produce melatonin when tryptophan was not added. After supplementation with the precursor, only Z. bailii and T. delbrueckii CBS1146 produced significant amounts of melatonin (37 and 30 ng/109 CFU, respectively) after 48 h, and, to a lesser extent, S cerevisiae UMY255 24 h after inoculation (14 ng/109 CFU). Five melatonin isomers were detected with different retention times (RT 1.91, 2.10, 2.61, 3.2, 3.41 min). Only T. delbrueckii CBS1146 produced significant levels of all five isomers. In all the samples, the isomer RT 2.61 was the most abundant and, in general, its concentration generally increased in tryptophan-enriched medium, reaching the highest levels in Z. bailii and T. delbrueckii CBS1146, 48 h after inoculation (439 and 1127 ng/109 CFU, respectively), and S cerevisiae UMY255 after 24 h (557 ng/109 CFU). The yeasts-mediated synthesis of melatonin after tryptophan supplementation seems to be a strainspecific character rather than a phenomenon related to the species. Further studies will investigate the contribute of indoleamines from microbial origin to the final levels of these metabolites in wine, as well as their biological role(s) and nutritional significance.