Relationship Between the Limonoid Content in Different Parts of the Sour Orange (Citrus aurantium L.) and the Ligand Activity of a Bile Acid Receptor, TGR5

抄録

Limonoids, a group of highly oxygenated triterpenoids mostly found in the Rutaceae and Meliaceae families, have many biological and physiological activities, such as anti-cancer, anti-microbial, and insecticidal ones. Recent studies suggest that some types of limonoids bind to a bile acid receptor, TGR5 (Takeda G protein–coupled receptor 5), and confer anti-obesity and anti-hyperglycemic effects. TGR5, also known as a G protein–coupled bile acid receptor 1 (GPBAR1), is a vital member of the membrane-bound G protein–coupled receptor (GPCR) family. In this study, we revealed the content of four types of limonoids (limonin, nomilin, obacunone, and limonin glucoside) and TGR5 ligand activity in a sour orange extract. The total concentration of the four limonoids was highest in the extract of ethyl acetate, followed by methanol and hexane in sour orange seeds. On the other hand, a luciferase assay using CHO cells transfected with a TGR5 confirmed that TGR5 ligand activity in the ethyl acetate extract of the seeds was as high as that in 50 μM nomilin, followed by that in the methanol extract of the seeds. The correlation coefficient between the limonoid content and the TGR5 ligand activity showed the highest value (r = 0.867) for nomilin, which supported a previous report that the TGR5 ligand activity of nomilin is higher than that of limonin or obacunone. However, the activity of the extract could not be explained by the nomilin content alone because the nomilin concentration in the extract used for the TGR5 assay was 3.9 μM, much lower than that in the control (50 μM nomilin), suggesting unknown compounds with higher TGR5 ligand activities in the seed extracts. In addition, the extract from cotyledons or germinated seeds showed higher TGR5 activity. Taken together, these results indicate that the seeds of citrus, such as the sour orange, may be a source of compounds that prevent obesity and metabolic disorders. In a future study, it will be necessary to comprehensively investigate citrus seed extracts to identify unidentified agonists for the TGR5 receptor.