Citrus Limonin Glucoside Supplementation Decreases Biomarkers of Disease and Inflammation in Humans

            It is commonly known that fresh fruits should be part of a healthy, balanced diet. Epidemiological studies reinforce this notion, revealing that increased consumption of fresh fruits is associated with reduced risk of diabetes, cardiovascular disease and cancer. Are there specific phytochemicals and biomolecules present in fruits, aside from well-known and well-studied flavonoids and vitamin C, that elicit these demonstrated health benefits in humans? Identification of fruit-derived nutraceutical components could provide access to new dietary supplements for prevention and/or treatment of various chronic diseases.

            Citrus limonoids are highly oxygenated triterpenoids that occur in a variety of citrus tissues in significant quantities (up to 900 ppm in orange juice). Limonoids exist in citrus juice and tissues as water-soluble glucosides. Citrus seeds and peel extract contain water-insoluble limonoid aglycones. Certain limonoid aglycones such as the flagship member of the limonoid natural product family, limonin, are responsible for the development of delayed bitterness in citrus. Limonin was isolated in the 1840’s but its precise chemical structure was not elucidated until 1960, when a collaborative team that included Derek Barton and E. J. Corey solved the structure using chemical derivatization and X-ray diffraction methods.

            A recent report from Darshan Kelley and co-workers at the USDA has demonstrated that consumption of a specific limonoid natural product, limonin glucoside (structure shown above), effectively reduces plasma concentrations of markers for chronic inflammatory diseases in human subjects. The authors examined the effects of limonin glucoside consumption on blood lipids, lipoproteins and liver enzymes. In this study, twelve-ounce drinks containing 250 milligrams of limonin glucoside dissolved in aqueous citrate buffer solution were consumed twice per day by subjects. The drinks were orange-flavored and contained some vitamin C and zero calories. The double blind study was also placebo controlled. Notably, at a dose of 500 mg/day, limonin glucoside had no specific adverse effects and the drinks were well-tolerated. Moreover, the drinks effectively reduced serum concentrations of several hepatic markers that are recognized to be associated with obesity and inflammation. The reduction in markers included gamma-glutamyl transferase (GGT, 34%), alanine aminotransferase (ALT, 13%) and alkaline phosphatase (ALP, 10%). Circulating concentrations of GGT, ALT and ALP are elevated in several human diseases including alcoholic and nonalcoholic fatty liver disease and metabolic syndrome. The authors note that one etiological link among these conditions is increased oxidative stress and inflammation and that future studies are warranted to examine the potential of limonoids to prevent or reverse these diseases.

            In forthcoming posts, we will examine the manner in which the authors of the study executed the deceptively challenging task of sourcing hundreds of grams of a highly complex natural product with purity in excess of 99.93%. We will also compare the USDA’s isolation method (extraction from citrus molasses) with the state-of-the-art in total chemical synthesis. The first total synthesis of racemic limonin was very recently disclosed by Shuji Yamashita’s and Masahiro Hirama’s research groups at Tohoku University (Japan). Limonin provides an outstanding case study by which to compare divergent supply chain approaches to sourcing structurally complex API’s.