Biochemistry and biological functions of citrus limonoids

Limonoids are one of bitter principles in citrus juices. Excessive bitterness lowers the quality and value of citrus juices. Nomilin is considered to be the precursor of all other limonoids accumulated in Citrus and related species. It is biosynthesized from acetate via the terpenoid biosynthetic pathway in the phloem region of stems and then translocated to the leaves, fruit tissues, peels, and seeds where it is further metabolized to other limonoids. The citrus limonoid aglycones are then glucosidated by limonoid UDP-D-glucose transferase in maturing fruit tissues and seeds. These limonoid glucosides are accumulated in such high concentrations that they are one of major secondary metabolites in citrus fruit tissues, and they play an important role in fruit quality and possibly in human health. Research on creation of transgenic citrus trees that produce fruits free of the limonoid bitterness problem is in progress. Limonoids have been shown to induce glutathione S-transferase activity and inhibit the formation of certain chemically induced carcinogenesis in laboratory animals. Some of the limonoids possess antifeedant activity against insects, suggesting that one of the biological functions of limonoids in plants is pest control. The citrus limonoids are unique for many species and varieties, and make excellent taxonomic markers. Thus, limonoids are important citrus constituents to citrus growers, processors, breeders, and the consumer.