Citrus limonoids: seasonal changes and their potential in glutathione S-transferase induction

Abstract

Seasonal changes of citrus limonids in 'Rio Red' grapefruit were investigated using reverse phase high performance liquid chromatography (HPLC). Several citrus limonoids, limonoid mixtures and a flavonone were evaluated for their ability to induce glutathione (GSH) and glutathione S-transferase (GST) in laboratory animals. In the flesh, segment membrane and albedo part of 'Rio Red' grapefruit, limonoate A-ring lactone (LARL) decreased, while limonin 17-b-D-glucopyranoside (LG) increased during late stages of fruit growth and maturation, suggesting that LARL was converted to LG as fruit maturation continued. Since the sudden decrease of LARL and the first appearance of LG did not take place simultaneously, there might be translocations of limonoids in forms other than nomilin among different tissues of 'Rio Red' grapefruit. In the seeds, both LARL and LG increased steadily throughout the experiment. Studies conducted on the effect of citrus limonoids on GSH level and GST activity showed that certain citrus limonoids had potential to induce both GSH level and GST activity. Limonoid aglycones were more effective than their glucosides as enzyme inducers. Nomilin and aglycone mixtures were the most potent inducers. The GST inducing activity of citrus limonoids appears to be structure-related. The opened D ring and/or the addition of a glucose molecule of the limonoid glucosides may be responsible for reduced GST-inducing acivity. An intact A ring of citrus limonoids seemed to be necessary for the enzyme induction. Further research is needed to explore the absorption, distribution and metabolism of citrus limonoids in animal systems.