Indole melatonin agonists and antagonists derived by shifting the melatonin side chain from the C-3 to the N-1 or to the C-2 indole position

Molecular Mechanisms Underlying the Biosynthesis of Melatonin and Its Isomer in Mulberry

Mulberry (Morus alba L.) leaves and fruit are traditional Chinese medicinal materials with anti-inflammatory, immune regulatory, antiviral and anti-diabetic properties. Melatonin performs important roles in the regulation of circadian rhythms and immune activities. We detected, identified and quantitatively analyzed the melatonin contents in leaves and mature fruit from different mulberry varieties. Melatonin and three novel isoforms were found in the Morus plants. Therefore, we conducted an expression analysis of melatonin and its isomer biosynthetic genes and in vitro enzymatic synthesis of melatonin and its isomer to clarify their biosynthetic pathway in mulberry leaves. MaASMT4 and MaASMT20, belonging to class II of the ASMT gene family, were expressed selectively in mulberry leaves, and two recombinant proteins that they expressed catalyzed the conversion of N-acetylserotonin to melatonin and one of three isomers in vitro. Unlike the ASMTs of Arabidopsis and rice, members of the three ASMT gene families in mulberry can catalyze the conversion of N-acetylserotonin to melatonin. This study provides new insights into the molecular mechanisms underlying melatonin and its isomers biosynthesis and expands our knowledge of melatonin isomer biosynthesis.

Microorganisms: Producers of Melatonin in Fermented Foods and Beverages

Melatonin has recently been detected in fermented beverages and foods, in which microorganism metabolism is highly important. The existing literature knowledge discusses the direction for future studies in this review. Evidence shows that many species of microorganisms could synthesize melatonin. However, the actual concentrations of melatonin in fermented foods and beverages range from picograms per milliliter to nanograms per milliliter. Different types of microorganisms, different raw materials, different culture environments, the presence or absence of precursors, high or low alcohol content, and different detection methods are all possible reasons for the huge difference of melatonin levels. Thus far, there have been relatively few studies on the melatonin synthesis pathway microorganisms. Thus, referring to the synthetic pathway of plants and animals, the putative melatonin biosynthesis pathway of microorganisms is presented. It will be significant to discuss whether all species of microorganisms have the capacity to synthesize melatonin and what the biological functions of melatonin are in microorganisms. Melatonin plays a lot of important roles in microorganisms, particularly in enhancing the tolerance of environment stress. Also, the loss of melatonin concentration in commercially available fermented foods and beverages is a ubiquitous trend, and how to solve this problem is a new field to be further explored.

Fundamental issues related to the origin of melatonin and melatonin isomers during evolution: relation to their biological functions

Melatonin and melatonin isomers exist and/or coexist in living organisms including yeasts, bacteria and plants. The levels of melatonin isomers are significantly higher than that of melatonin in some plants and in several fermented products such as in wine and bread. Currently, there are no reports documenting the presence of melatonin isomers in vertebrates. From an evolutionary point of view, it is unlikely that melatonin isomers do not exist in vertebrates. On the other hand, large quantities of the microbial flora exist in the gut of the vertebrates. These microorganisms frequently exchange materials with the host. Melatonin isomers, which are produced by these organisms inevitably enter the host's system. The origins of melatonin and its isomers can be traced back to photosynthetic bacteria and other primitive unicellular organisms. Since some of these bacteria are believed to be the precursors of mitochondria and chloroplasts these cellular organelles may be the primary sites of melatonin production in animals or in plants, respectively. Phylogenic analysis based on its rate-limiting synthetic enzyme, serotonin N-acetyltransferase (SNAT), indicates its multiple origins during evolution. Therefore, it is likely that melatonin and its isomer are also present in the domain of archaea, which perhaps require these molecules to protect them against hostile environments including extremely high or low temperature. Evidence indicates that the initial and primary function of melatonin and its isomers was to serve as the first-line of defence against oxidative stress and all other functions were acquired during evolution either by the process of adoption or by the extension of its antioxidative capacity.

Melatonin, melatonin isomers and stilbenes in Italian traditional grape products and their antiradical capacity

Although polyphenols represent the paradigm of the health-promoting effects ascribed to grape products, recently, attention has been paid to dietary melatonin, significantly present in Mediterranean foods. In this work, we measured melatonin, its isomers, stilbenes (trans- and cis-resveratrol and their glucosides, piceids) and total polyphenols in some different grape products (red, white and dessert wines, grape juices and Modena balsamic vinegars) of distinct Italian areas. We also evaluated their antiradical activity by DPPH(·) and ABTS(·+) assays. For indoleamine analysis, the separation was carried out on a 1.7-μm C18 BEH column and the detection performed by means of mass spectrometry with electrospray ionization in positive ion mode with multiple reaction monitoring. The confirmation of the peak identity was accomplished by injection into the high-resolution system (Orbitrap) using accurate mass measurements (error below 1.0 ppm). Mass spectrometry analyses revealed, for the first time, the presence of melatonin in dessert wines and balsamic vinegars, as well as the occurrence of three different melatonin isomers in grape products.

Emergence of naturally occurring melatonin isomers and their proposed nomenclature

Melatonin was considered to be the sole member of this natural family. The emergence of naturally occurring melatonin isomers (MIs) has opened an exciting new research area. Currently, several MIs have been identified in wine, and these molecules are believed to be synthesized by either yeasts or bacteria. A tentative nomenclature for the MIs is proposed in this article. It will be important to explore whether all organisms have the capacity to synthesize MIs, especially under the conditions of environmental stress. These isomers probably share many of the biological functions of melatonin, but their activities seem to exceed those of melatonin. On basis of the limited available information, it seems that MIs differ in their biosynthetic pathways from melatonin. Especially in those compounds in which the aliphatic side chain is not attached to ring atom 3, the starting material may not be tryptophan. Also, the metabolic pathways of MIs remain unknown. This, therefore, is another promising area of research to explore. It is our hypothesis that MIs would increase the performance of yeasts and probiotic bacteria during the processes of fermentation. Therefore, yeasts producing elevated levels of these isomers might have a superior alcohol tolerance and be able to produce higher levels of alcohol. This can be tested by comparing existing yeast strains differing in alcohol tolerance. Selection for MIs may become a strategy for isolating more resistant yeast and Lactobacillus strains, which can be of interest for industrial alcohol production and quality improvements in bacterially fermented foods such as kimchi.