N-methylated derivatives of tyramine in citrus genus plants: identification of N,N,N-trimethyltyramine (candicine)

Amino acids, betaines and related ammonium compounds in Neapolitan limmo, a Mediterranean sweet lime, also known as lemoncetta Locrese

BACKGROUND

The so-called 'Neapolitan limmo' or 'lemoncetta Locrese' is an old and now rare Mediterranean sweet lime, similar to lemon but smaller. It is a fruit distinguished from orange, lemon, mandarin, and lime for its sweeter, watery, and non-acidic taste, with a pH between 5.6 and 5.9. No compositional studies are currently available for this citrus fruit. Here we report, for the first time, the distribution in the limmo juice of free amino acids and their main derivatives such as betaines and related ammonium compounds.

RESULTS

Seven proteinogenic amino acids (proline, asparagine, serine, aspartic acid, glutamine, alanine, and threonine) and a non-protein amino acid (γ-aminobutyric acid) characterize Neapolitan limmo juice. Proline betaine is the predominant betaine. The data were compared with those of other important citrus juices.

CONCLUSION

The specific 'taste quality' of Neapolitan limmo juice can be attributed to its peculiar composition in amino acids. The species-specific presence of the ammonium compound derivatives of the amino acid proline, with proline betaine as the predominant betaine, characterize the non-acidic varieties of Mediterranean sweet lime. Our study constitutes an important step towards the repopulation of this ancient plant and its exploitation in food industry. © 2020 Society of Chemical Industry.

Absorption, Biokinetics, and Metabolism of the Dopamine D2 Receptor Agonist Hordenine (N,N-Dimethyltyramine) after Beer Consumption in Humans

Hordenine, a natural constituent of germinated barley, is a biased agonist of the dopamine D2 receptor. This pilot study investigated the biokinetics of hordenine and its metabolites in four volunteers consuming beer equal to 0.075 mg hordenine/kg body weight. A new ultrahigh-performance liquid chromatography method coupled to electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS/MS) method determined maximum plasma concentrations of 12.0-17.3 nM free hordenine after 0-60 min. Hordenine phase-II metabolism was first dominated by sulfation, but later by glucuronidation. The elimination half-lives in plasma were 52.7-66.4 min for free hordenine and about 60/80 min longer for hordenine sulfate and hordenine glucuronide. Urinary excretion peaked 2-3.5 h after consumption and accumulated to 3.78 μmol within 24 h, corresponding to 9.9% of the ingested dose. The observed hordenine levels in plasma seem too low to provoke direct interaction with the dopamine D2 receptor related to food reward, but synergistic or additive effects with alcohol or N-methyltyramine may occur.

Structure and Ligands Interactions of Citrus Tryptophan Decarboxylase by Molecular Modeling and Docking Simulations

In a previous work, we in silico annotated protein sequences of Citrus genus plants as putative tryptophan decarboxylase (pTDC). Here, we investigated the structural properties of Citrus pTDCs by using the TDC sequence of Catharanthus roseus as an experimentally annotated reference to carry out comparative modeling and substrate docking analyses. The functional annotation as TDC was verified by combining 3D molecular modeling and docking simulations, evidencing the peculiarities and the structural similarities with C. roseus TDC. Docking with l-tryptophan as a ligand showed specificity of pTDC for this substrate. These combined results confirm our previous in silico annotation of the examined protein sequences of Citrus as TDC and provide support for TDC activity in this plant genus.

Effect of N-methyltyramine on the regulation of adrenergic receptors via enzymatic epinephrine synthesis for the treatment of gastrointestinal disorders

BACKGROUND

Citri Reticulatae Pericarpium (CRP), Aurantii Fructus Immaturus (AFI) and Aurantii Fructus (AF) are all important Citrus species used in traditional Chinese medicines (TCMs) for the treatment of gastrointestinal disorders. Although they have been used since ancient times and are still in use today, the mechanistic basis for their regulation of adrenergic receptors (ARs) is still not clear.

PURPOSE

In this study, we aimed to determine the active components and mechanisms of action of CRP, AFI and AF in treating gastrointestinal disorders related to ARs.

METHODS

First, the phenethylamine alkaloid components of CRP, AFI and AF were identified and compared across 30 samples of three Citrus species by UPLC-Q/TOF-MS in combination with content difference analysis. Second, the effect of the main active alkaloid component on AR-based gastrointestinal disorders was investigated by an in vivo small intestinal propulsive test and an in vitro relaxing small intestinal smooth muscle activity test. The mechanism of AR regulation of the active alkaloid was further studied by evaluating its effect on relaxing small intestinal smooth muscle in the presence of an inhibitor. Lastly, the enzymes, which played an important role in epinephrine synthesis and AR regulation, were detected by immunohistochemistry.

RESULTS

Three phenethylamine AR regulators (N-methyltyramine, synephrine and hordenine) in CRP, AFI and AF were characterized. It was found that N-methyltyramine could relax mouse small intestinal smooth muscle and inhibit small intestinal propulsion. The effect of N-methyltyramine on relaxing small intestinal smooth muscle could be inhibited by a-methyl-l-tyrosine. The enzymes related epinephrine synthesis and AR function were found in the mouse small intestine. The biotransformation process that converts N-methyltyramine to epinephrine was determined.

CONCLUSION

The treatment of gastrointestinal disorders of CRP, AFI and AF is associated with their alkaloid component N-methyltyramine via the regulation of ARs, and the mechanism is considered to be the biotransformation of N-methyltyramine to epinephrine by serial synthase, which takes place at the nerves cells in small intestine.

Elucidating the mechanism behind the laccase-mediated modification of poly(ethersulfone)

Laccase-mediated oligomerisation of 4-hydroxybenzoic acid (4-HBA) derivatives and simultaneous in situ surface modification has proven to be a cost-effective, easily applicable and eco-friendly strategy for preventing biofouling of poly(ethersulfone) (PES) water filtration membranes. Modification of the membrane surface has previously been hypothesised to occur through covalent bonding of enzymatically generated phenolic radicals to the polymeric membrane. The current study shows, however, that in situ formation of soluble phenolic oligomers does not result in covalent membrane modification. We studied in situ laccase-mediated oligomerisation of custom-synthesised positively charged and commercially available negatively charged monomeric phenols, and demonstrated that their mode of binding to PES is not covalent. In addition, soluble, non-soluble and on-resin PES model compounds were synthesised and used in the laccase-mediated oligomerisation of 4-HBA. Covalent bond formation between these model compounds and (oligomeric) 4-HBA could not be observed either. Furthermore, extensive washing of PES membranes modified through laccase-mediated oligomerisation of 4-HBA resulted in substantial discolouration of the membrane surface, showing that the layer of oligomerised phenolics could easily be removed. Altogether, it was concluded that laccase-assisted modification of PES membranes resulted from strong physical adsorption of phenolic oligomers and polymers rather than from covalent bonding of those.

The mechanism behind the laccase-mediated functionalisation of poly(ethersulfone) was studied using a multifaceted approach, which revealed that surface modification had occurred through strong physical adsorption, rather than through grafting of phenolic oligomers.

Experimental Evidence and In Silico Identification of Tryptophan Decarboxylase in Citrus Genus

Plant tryptophan decarboxylase (TDC) converts tryptophan into tryptamine, precursor of indolealkylamine alkaloids. The recent finding of tryptamine metabolites in Citrus plants leads to hypothesize the existence of TDC activity in this genus. Here, we report for the first time that, in Citrus x limon seedlings, deuterium labeled tryptophan is decarboxylated into tryptamine, from which successively deuterated N,N,N-trimethyltryptamine is formed. These results give an evidence of the occurrence of the TDC activity and the successive methylation pathway of the tryptamine produced from the tryptophan decarboxylation. In addition, with the aim to identify the genetic basis for the presence of TDC, we carried out a sequence similarity search for TDC in the Citrus genomes using as a probe the TDC sequence reported for the plant Catharanthus roseus. We analyzed the genomes of both Citrus clementina and Citrus sinensis, available in public database, and identified putative protein sequences of aromatic l-amino acid decarboxylase. Similarly, 42 aromatic l-amino acid decarboxylase sequences from 23 plant species were extracted from public databases. Potential sequence signatures for functional TDC were then identified. With this research, we propose for the first time a putative protein sequence for TDC in the genus Citrus.

Tyramine Pathways in Citrus Plant Defense: Glycoconjugates of Tyramine and Its N-Methylated Derivatives

Glucosylated forms of tyramine and some of its N-methylated derivatives are here reported for the first time to occur in Citrus genus plants. The compounds tyramine-O-β-d-glucoside, N-methyltyramine-O-β-d-glucoside, and N,N-dimethyltyramine-O-β-d-glucoside were detected in juice and leaves of sweet orange, bitter orange, bergamot, citron, lemon, mandarin, and pomelo. The compounds were identified by mass spectrometric analysis, enzymatic synthesis, and comparison with extracts of Stapelia hirsuta L., a plant belonging to the Apocynaceae family in which N,N-dimethyltyramine-O-β-d-glucoside was identified by others. Interestingly, in Stapelia hirsuta we discovered also tyramine-O-β-d-glucoside, N-methyltyramine-O-β-d-glucoside, and the tyramine metabolite, N,N,N-trimethyltyramine-O-β-glucoside. However, the latter tyramine metabolite, never described before, was not detected in any of the Citrus plants included in this study. The presence of N-methylated tyramine derivatives and their glucosylated forms in Citrus plants, together with octopamine and synephrine, also deriving from tyramine, supports the hypothesis of specific biosynthetic pathways of adrenergic compounds aimed to defend against biotic stress.

Serotonin 5-O-β-Glucoside and Its N-Methylated Forms in Citrus Genus Plants

Citrus genus is characterized by a specific presence of indole metabolites deriving from the N-methylation of tryptamine and its hydroxylated form, 5-hydroxytryptamine (serotonin), which are likely involved in plant defense mechanisms. In this study, we identified for the first time the occurrence in Citrus plants of serotonin 5-O-β-glucoside and all its N-methylated derivatives, that is, N-methylserotonin 5-O-β-glucoside, N,N-dimethylserotonin (bufotenine) 5-O-β-glucoside, and N,N,N-trimethylserotonin (bufotenidine) 5-O-β-glucoside. The identification of the glucosylated compounds was based on mass spectrometric studies, hydrolysis by glucosidase, and in some cases, comparison to authentic compounds. Beside leaves, the distribution of the glucosylated forms and their aglycones in some Citrus species was evaluated in flavedo, albedo, juice, and seeds. The simultaneous presence of serotonin and its N-methylated derivatives, together with the corresponding glucosylated forms, is consistent with the occurrence of a metabolic pathway, specific for Citrus, aimed at potentiating the defensive response to biotic stress through the optimization of the production and use of the most toxic of such metabolites.