Yusuke Amino

Involvement of the calcium-sensing receptor in human taste perception.

By human sensory analyses, we found that various extracellular calcium-sensing receptor (CaSR) agonists enhance sweet, salty, and umami tastes, although they have no taste themselves. These characteristics are known as “kokumi taste” and often appear in traditional Japanese cuisine. Although GSH is a typical kokumi taste substance (taste enhancer), its mode of action is poorly understood. Here, we demonstrate how the kokumi taste is enhanced by the CaSR, a close relative of the class C G-protein-coupled receptors T1R1, T1R2, and T1R3 (sweet and umami receptors). We identified a large number of CaSR agonist γ-glutamyl peptides, including GSH (γ-Glu-Cys-Gly) and γ-Glu-Val-Gly, and showed that these peptides elicit the kokumi taste. Further analyses revealed that some known CaSR agonists such as Ca2+, protamine, polylysine, l-histidine, and cinacalcet (a calcium-mimetic drug) also elicit the kokumi taste and that the CaSR-specific antagonist, NPS-2143, significantly suppresses the kokumi taste. This is the first report indicating a distinct function of the CaSR in human taste perception.

Synthesis of 2′,3′-Dideoxypurinenucleosides via the Palladium Catalyzed Reduction of 9-(2,5-Di-O-acetyl-3-bromo-3-deoxy-β-d-xylofuranosyl)purine Derivatives †

Abstract Practical method to produce 2′,3′-dideoxypurinenucleosides from 9-(2,5-di-O-acetyl-3-bromo-3-deoxy-β-D-xylofuranosyl)purines (1) was developed. High ratio of 2′,3′-dideoxynucleoside to 3′-deoxyribonucleoside was obtained by selecting the reaction conditions (solvent, pH and/or base), or changing 2′-acyloxy leaving group. The reaction mechanism was studied by deuteration experiments of 1a and 1-(3,5-di-O-acety1-2-bromo-2-deoxy-β-D-ribofuranosyl)thymine (12). †Dedicated to Dr. Yoshihisa Mizuno on the occasion of his 75th birthday.

Conformational analysis of the dipeptide taste ligandL-aspartyl-D-2-aminobutyric acid-(S)-α-ethylbenzylamide and its analogues by NMR spectroscopy, computer simulations and X-ray diffraction studies

A dipeptide taste ligand L‐aspartyl‐D‐2‐aminobutyric acid‐(S)‐α‐ethylbenzylamide was found to be about 2000 times more potent than sucrose. To investigate the molecular basis of its potent sweet taste, we carried out conformational analysis of this molecule and several related analogues by NMR spectroscopy, computer simulations and X‐ray crystallographic studies. The results of the studies support our earlier model that an ‘L’‐shape molecular array is essential for eliciting sweet taste. In addition, we have identified an aromatic group located between the stem and the base of the ‘L‐shape’, which is responsible for enhancement of sweetness potency. In this study, we also assessed the optimal size of the essential hydrophobic group (X) and the effects of the chirality of the second residue toward taste. ©1997 European Peptide Society and John Wiley & Sons, Ltd.

Conformation Analysis of Aspartame-Based Sweeteners by NMR Spectroscopy, Molecular Dynamics Simulations, and X-ray Diffraction Studies

We report here the synthesis and the conformation analysis by 1H NMR spectroscopy and computer simulations of six potent sweet molecules, N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)‐3‐methylbutyl]‐α‐L‐aspartyl‐S‐tert‐butyl‐L‐cysteine 1‐methylester (1; 70 000 times more potent than sucrose), N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)‐3‐methylbutyl]‐α‐L‐aspartyl‐β‐cyclohexyl‐L‐alanine 1‐methylester (2; 50 000 times more potent than sucrose), N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)‐3‐methylbutyl]‐α‐L‐aspartyl‐4‐cyan‐L‐phenylalanine 1‐methylester (3; 2 000 times more potent than sucrose), N‐[3,3‐dimethylbutyl]‐α‐L‐aspartyl‐(1R,2S,4S)‐1‐methyl‐2‐hydroxy‐4‐phenylhexylamide (4; 5500 times more potent than sucrose), N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)propyl]‐α‐L‐aspartyl‐(1R,2S,4S)‐1‐methyl‐2‐hydroxy‐4‐phenylhexylamide (5; 15 000 times more potent than sucrose), and N‐[3‐(3‐hydroxy‐4‐methoxyphenyl)‐3‐methylbutyl]‐α‐L‐aspartyl‐(1R,2S,4S)‐1‐methyl‐2‐hydroxy‐4‐phenylhexylamide (6; 15 000 times more potent than sucrose). The “L‐shaped” structure, which we believe to be responsible for sweet taste, is accessible to all six molecules in solution. This structure is characterized by a zwitterionic ring formed by the AH‐ and B‐containing moieties located along the +y axis and by the hydrophobic group X pointing into the +x axis. Extended conformations with the AH‐ and B‐containing moieties along the +y axis and the hydrophobic group X pointing into the −y axis were observed for all six sweeteners. For compound 5, the crystal‐state conformation was also determined by an X‐ray diffraction study. The result indicates that compound 5 adopts an L‐shaped structure even in the crystalline state. The extraordinary potency of the N‐arylalkylated or N‐alkylated compounds 1–6, as compared with that of the unsubstituted aspartame‐based sweet taste ligands, can be explained by the effect of a second hydrophobic binding domain in addition to interactions arising from the L‐shaped structure. In our examination of the unexplored D zone of the Tinti–Nofre model, we discovered a sweet‐potency‐enhancing effect of arylalkyl substitution on dipeptide ligands, which reveals the importance of hydrophobic (aromatic)–hydrophobic (aromatic) interactions in maintaining high potency.

Structure–CaSR–Activity Relation of Kokumi γ-Glutamyl Peptides

Modulation of the calcium sensing receptor (CaSR) is one of the physiological activities of γ-glutamyl peptides such as glutathione (γ-glutamylcysteinylglycine). γ-Glutamyl peptides also possess a flavoring effect, i.e., sensory activity of kokumi substances, which modifies the five basic tastes when added to food. These activities have been shown to be positively correlated, suggesting that kokumi γ-glutamyl peptides are perceived through CaSRs in humans. Our research is based on the hypothesis that the discovery of highly active CaSR agonist peptides will lead to the creation of practical kokumi peptides. Through continuous study of the structure-CaSR-activity relation of a large number of γ-glutamyl peptides, we have determined that the structural requirements for intense CaSR activity of γ-glutamyl peptides are as follows: existence of an N-terminal γ-L-glutamyl residue; existence of a moderately sized, aliphatic, neutral substituent at the second residue in an L-configuration; and existence of a C-terminal carboxylic acid, preferably with the existence of glycine as the third constituent. By the sensory analysis of γ-glutamyl peptides selected by screening using the CaSR activity assay, γ-glutamylvalylglycine was found to be a potent kokumi peptide. Furthermore, norvaline-containing γ-glutamyl peptides, i.e., γ-glutamylnorvalylglycine and γ-glutamylnorvaline, possessed excellent sensory activity of kokumi substances. A novel, practical industrial synthesis of regiospecific γ-glutamyl peptides is also required for their commercialization, which was achieved through the ring opening reaction of N-α-carbobenzoxy-L-glutamic anhydride and amino acids or peptides in the presence of N-hydroxysuccinimide.

Preparation and Characterization of Four Stereoisomers of Monatin.

Monatin is a naturally occurring, sweet amino acid comprising four stereoisomers due to its two asymmetric centers at C2 and C4. However, the characteristics of each stereoisomer have not yet been fully investigated. To obtain a sufficient amount of racemic monatin for optical resolution, a synthetic method was developed by modifying a possible biosynthetic pathway, i.e., a cross-aldol reaction and subsequent transamination. The key intermediate, 4-hydroxy-4-(3-indolylmethyl)-2-ketoglutaric acid, was obtained via the cross-aldol reaction of pyruvic acid and indole-3-pyruvic acid. Subsequently, the carbonyl group was converted to a hydroxyimino group through reaction with hydroxylamine and then to an amino group via hydrogenation to produce monatin. Next, the racemic monatin was divided into mixtures of two pairs of enantiomers through recrystallization. Finally, both enantiomers of the N-carbobenzoxy-γ-lactone derivatives of monatin were separated by preparative HPLC and deprotected. It was found that all optically pure stereoisomers exhibited a sweet taste. The isomer that displayed the most intense sweetness was the (2R,4R)-isomer, as determined by single crystal X-ray structure analysis of the monatin potassium salt, whereas the least sweet isomer was the (2S,4S)-isomer, which demonstrated a far lower sweetness than was previously reported.

Stereo-Selective Preparation of Teneraic Acid, trans -(2 S ,6 S )-Piperidine-2,6-dicarboxylic Acid, via Anodic Oxidation and Cobalt-Catalyzed Carbonylation

Teneraic acid (piperidine-2,6-dicarboxylic acid) is a naturally occurring imino acid that comprises three stereoisomers due to its two asymmetric centers at C2 and C6. The configuration of natural teneraic acid is reported to correspond to trans-(2S,6S). However, a few studies are focused on the stereospecific synthesis of trans-(2S,6S)-teneraic acid. The present study investigates a convenient synthetic method that includes regiospecific anodic oxidation and stereospecific cobalt-catalyzed carbonylation to obtain trans-(2S,6S)-teneraic acid. Methyl (S)-N-benzoyl-α-methoxypipecolate, the key intermediate that displays a structure that corresponds to an intermediate (N-α-hydroxyalkyl amide) of intramolecular amidocarbonylation, was obtained via an anodic oxidation of methyl (S)-N-benzoylpipecolate. Subsequently, cobalt-catalyzed carbonylation converted the methyl (S)-N-benzoyl-α-methoxypipecolate to trans-(2S,6S)-N-benzoyl-teneraic acid dimethyl ester in good optical purity (>95% enantiomeric excess (ee)) and modest yield (63%). Finally, de-protection occurred via acidic hydrolysis to obtain trans-(2S,6S)-teneraic acid. The stereochemistry of synthesized teneraic acid was confirmed as corresponding to trans-(2S,6S) by comparing its physical properties with those of a cis-meso-isomer and those of a trans-(2S,6S)-isomer that were reported in previous studies.

Practical large-scale production of dihydrocapsiate, a nonpungent capsaicinoid-like substance

Capsinoids represent a novel group of capsaicinoid-like substances found in a nonpungent cultivar, Capsicum annuum “CH-19 Sweet.” They have capsaicinoid-like physiological and biological properties while lacking the harmful stimuli of capsaicinoids. A large-scale synthesis of dihydrocapsiate (DCT) is established in this work. 8-Methynonanoic acid (MNA) was synthesized by copper-catalyzed cross-coupling of ethyl 6-bromohexanoate with isobutylmagnesium bromide and subsequent hydrolysis. Lipase-catalyzed chemoselective esterification of vanillyl alcohol and MNA was performed at 50 °C under reduced pressure to remove water without solvents or drying agents. A slightly larger stoichiometric amount of MNA was used and the purification in the final stage was simplified to leave a small amount of MNA in the product, because we found that the presence of a small amount of MNA is necessary to stabilize DCT. DCT was synthesized according to the production, and stabilization methods described here has been filed as a new dietary ingredient. Dihydrocapsiate was synthesized via lipase-catalyzed chemoselective esterification of vanillyl alcohol and 8-methylnonanoic acid under reduced pressure without solvents.

Concise Synthesis of (2R,4R)-Monatin

Monatin, 4-hydroxy-4-(3-indolylmethyl)-glutamic acid, is a naturally occurring sweet amino acid. The (2R,4R)-monatin isomer has been found to be the sweetest among its four stereoisomers. A concise and efficient synthesis of (2R,4R)-monatin was accomplished by the alkylation of (4R)-N-tert-butoxycarbonyl (tBoc)-4-tert-butyldimethylsilyoxy-D-pyroglutamic acid methyl ester with tert-butyl 3-(bromomethyl)-1H-indole-1-carboxylate to give (4R)-N-tBoc-4-tert-butyldimethylsilyloxy-4-(N-tBoc-3-indolylmethyl)-D-pyroglutamic acid methyl ester, i.e., the lactam form of (2R,4R)-monatin with protecting groups. This was followed by the hydrolysis of the lactam ring and deprotection. The 4-hydroxyl D-pyroglutamic acid derivative was demonstrated to be a suitable precursor for the efficient preparation of (2R,4R)-monatin in high optical purity because the alkylation proceeded in regioselective and stereoselective manners at C4 to form appropriate asymmetric tetra-substituted carbon center; the resulting alkylated pyroglutamic acid derivative was then easily converted into the linear form of monatin.

Human–Environment Interactions – Taste

Humans have evolved to consume natural products in their environment and have acquired a sense of taste. Our ability to taste bitter and sour evolved to identify potentially dangerous food. On the contrary, the ability to recognize a sweet taste developed to identify an energy source, while a salty taste is a signal of minerals. As time progressed, humans began to use bitter, pungent, and even astringent tastes that were initially considered to be unpleasant. This may have been because we became aware that such substances were effective at improving health or even treating disease. Human food culture progressed further with the development of cooking methods that use spices and more dramatically through the enjoyment of fermentation products. There has been no recent comprehensive review on the chemistry of taste written from the perspective of natural products chemistry. In this chapter, several taste sensations found in natural products are described along with their structures. Although it is still very difficult to anticipate the taste quality and intensity from the structure of an organic compound, it is expected that recent progress in the study of receptors will contribute to a full understanding of the relationship between taste sensation and chemical structure.