Isolation and characterization of new limonoid glycosides from Citrus unshiu peels

Five New Limonoids from Peels of Satsuma Orange (Citrus reticulata)

Five new: 21,23-dihydro-21-hydroxy-23-oxonomilin (1), 21,23-dihydro-23-methoxy-21-oxonomilin (2), 21,23-dihydro-21-hydroxy-23-oxonomilinic acid methyl ester (3), 21,23-dihydro-23-methoxy-21-oxolimonin (4), and 21,23-dihydro-21-oxolimonin (5), and seven known limonoids were isolated from peels of satsuma orange (Citrus reticulata). The isolated compounds were evaluated for their inhibitory effects on macrophage activation by an inhibitory assay of nitric oxide (NO) production. Among them, compound (2) exhibited NO inhibitory activity without cytotoxicity.

Anti-inflammatory activity of a new cyclic peptide, citrusin XI, isolated from the fruits of Citrus unshiu

ETHNOPHARMACOLOGICAL RELEVANCE

Citrus unshiu (Rutaceae) is an easy-peeling citrus fruit, which has been used as a traditional Korean medicine for improving skin elasticity, relieving fatigue and cough, and preventing bronchitis, flu, and various cancers. However, its active components associated with anti-inflammation and underlying mechanisms remain unknown. In this study, we investigated the active constituents from the fruits of Citrus unshiu and evaluated the anti-inflammatory activity in order to support the traditional usage of Citrus unshiu.

MATERIAL AND METHODS

Repeated column chromatography, together with a semi-preparative HPLC purification was used to separate the bioactive constituent from the EtOAc soluble fraction of the EtOH extract of Citrus unshiu fruits. Anti-inflammatory effects of the isolated compounds on lipopolysaccharide (LPS)-induced production of pro-inflammatory mediators were examined using RAW264.7 macrophage cells.

RESULTS

A new cyclic peptide, citrusin XI (1), was isolated and identified from the fruits of Citrus unshiu. The structure of compound 1 was elucidated by spectroscopic analysis, including 1D and 2D nuclear magnetic resonance (NMR) ((1)H, (13)C, COSY, HMQC and HMBC experiments), and high resolution (HR)-mass spectrometry, and its absolute configurations were further confirmed by the Marfey׳s method. Compound 1 decreased NO production in LPS-stimulated RAW264.7 cells in a dose-dependent manner with an IC50 value of 70μM. Compound 1 suppressed NO production by decreasing iNOS expression but COX-2 expression was slightly associated with the reduction by compound 1 in LPS-induced RAW264.7 cells. Furthermore, compound 1 inhibited NF-κB activation by blocking IκBα degradation and NF-κB phosphorylation in LPS-stimulated RAW264.7 cells.

CONCLUSIONS

These results indicate that a new cyclic peptide, citrusin XI, from Citrus unshiu fruits has anti-inflammatory properties that inhibit the release of pro-inflammatory mediators. Compound 1 decreases NO production by decreasing iNOS expression and NF-κB activation associated with IκBα degradation and NF-κB phosphorylation in LPS-induced RAW264.7 cells. This is the first study to clarify the underlying mechanism of the anti-inflammatory effect exerted by a pure isolated compound from Citrus unshiu in LPS-stimulated RAW264.7 macrophage cells. The phytochemical, citrusin XI of Citrus unshiu may serve as lead compound in the design of new agents for preventing and treating inflammatory diseases.

Citrus limonoids: analysis, bioactivity, and biomedical prospects

Limonoids are a prominent group of secondary metabolites in citrus fruit. The bitter character of some compounds in this group has historically compromised the quality of citrus fruit and juice. Detecting bitter limonoids in citrus, understanding their origins, and developing methods for their removal from citrus juices have provided the basis for citrus limonoid research. Evaluation of the biological activity of citrus limonoids has indicated the potential of these compounds to improve human health as anticancer, cholesterol-lowering, and antiviral agents. This review chronicles the evolution of citrus limonoid research from defining their participation in citrus bitterness to their potential utilization as important contributors to improving human health and well-being.

Limonoids: overview of significant bioactive triterpenes distributed in plants kingdom

The search for limonoids started long back when scientists started looking for the factor responsible for bitterness in citrus which has negative impact on citrus fruit and juice industry worldwide. The term limonoids was derived from limonin, the first tetranortriterpenoid obtained from citrus bitter principles. Compounds belonging to this group have exhibited a range of biological activities like insecticidal, insect antifeedant and growth regulating activity on insects as well as antibacterial, antifungal, antimalarial, anticancer, antiviral and a number of other pharmacological activities on humans. Although hundreds of limonoids have been isolated from various plants but, their occurrence in the plant kingdom is confined to only plant families of order Rutales and that too more abundantly in Meliaceae and Rutaceae, and less frequently in Cneoraceae and Harrisonia sp. of Simaroubaceae. Limonoids are highly oxygenated, modified terpenoids with a prototypical structure either containing or derived from a precursor with a 4,4,8-trimethyl-17-furanylsteroid skeleton. All naturally occurring citrus limonoids contain a furan ring attached to the D-ring, at C-17, as well as oxygen containing functional groups at C-3, C-4, C-7, C-16 and C-17. The structural variations of limonoids found in Rutaceae are less than in Meliaceae and are generally limited to the modification of A and B rings, the limonoids of Meliaceae are more complex with very high degree of oxidation and rearrangement exhibited in the parent limonoid structure. To counter the problem of bitterness in citrus juice and products genetic engineering of citrus to maximize the formation of limonoid glucosides for reducing limonoid bitterness is the focus of recent and future research. Regarding the biological activities of limonoids the investigations are to be directed towards detailed characterization, quantification, and designing a simple as well as versatile synthetic route of apparently important limonoids. Extraction methods too should be optimized; evaluation and establishment of pharmaco-dynamic and kinetic principles, and structure activity relationships should be a key goal associated with limonoids so that they can be safely introduced in our arsenal of pharmaceuticals to safeguard the humanity from the wrath of disease and its discomfort.