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 ␥-glutamyl peptides, including GSH (␥-Glu-Cys-Gly) and ␥-Glu-Val-Gly, and showed that these peptides elicit the kokumi taste. Further analyses revealed that some known CaSR agonists such as Ca 2؉ , 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.

The effects of defatted safflower seed extract and its phenolic constituents, serotonin derivatives, on atherosclerosis were studied. Ethanol-ethyl acetate extract of safflower seeds (SSE) inhibited low-density lipoprotein (LDL) oxidation induced in vitro by an azo-containing free-radical initiator V70 or copper ions. Two serotonin derivatives [N-(p-coumaroyl)serotonin, CS; N-feruloylserotonin, FS] and their glucosides were identified as the major phenolic constituents of the extract. The study with chemically synthesized materials revealed that a majority of the antioxidative activity of SSE was attributable to the aglycones of these two serotonin derivatives. Orally administered CS and FS suppressed CuSO(4)-induced plasma oxidation ex vivo. Long-term (15 week) dietary supplementation of SSE (1.0 wt %/wt) and synthetic serotonin derivatives (0.2-0.4%) significantly reduced the atherosclerotic lesion area in the aortic sinus of apolipoprotein E-deficient mice (29.2-79.7% reduction). The plasma level of both lipid peroxides and anti-oxidized LDL autoantibody titers decreased concomitantly with the reduction of lesion formation. Serotonin derivatives were detected as both intact and conjugated metabolites in the plasma of C57BL/6J mice fed on 1.0% SSE diet. These findings demonstrate that serotonin derivatives of SSE are absorbed into circulation and attenuate atherosclerotic lesion development possibly because of the inhibition of oxidized LDL formation through their strong antioxidative activity.

No abstract

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 Nhydroxysuccinimide.

The structures, flavor-modifying effects, and CaSR activities of γ-glutamyl peptides comprising sulfur-containing amino acids were investigated. The chemical structures, including the linkage mode of the N-terminal glutamic acid, of γ-L-glutamyl-S-(2-propenyl)-L-cysteine (γ-L-glutamyl-S-allyl-L-cysteine) and its sulfoxide isolated from garlic were established by comparing their NMR spectra with those of authentic peptides prepared using chemical methods. Mass spectrometric analysis also enabled determination of the linkage modes in the glutamyl dipeptides by their characteristic fragmentation. In sensory evaluation, these peptides exhibited flavor-modifying effects (continuity) in umami solutions less pronounced but similar to that of glutathione. Furthermore, the peptides exhibited intrinsic flavor due to the sulfur-containing structure, which may be partially responsible for their flavor-modifying effects. In CaSR assays, γ-L-glutamyl-S-methyl-L-cysteinylglycine was most active, which indicates that the presence of a medium-sized aliphatic substituent at the second amino acid residue in γ-glutamyl peptides enhances CaSR activity.