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.