Chapter 12 - Oleuropeic and Menthiafolic Acid Glucose Esters from Plants: Shared Structural Relationships and Biological Activities

Abstract

Glycosylation of small lipophilic molecules is a ubiquitous process in plants that produces many important compounds. Many of these are secondary metabolites with key roles in plant abiotic and biotic stress tolerance through such processes as free radical scavenging and antimicrobial and antiherbivore defense. Most of these plant products occur as O -glycosides, with a much smaller number occurring as glucose esters of aromatic acids, and even fewer occurring as glucose esters of aliphatic acids. For example, there is a large diversity and distribution of monoterpene glycosides in plants, but relatively few monoterpene acid glucose esters have been elucidated to date. There have, however, been increasing reports of the latter esters in recent years, particularly those based on the isomers menthiafolic acid and oleuropeic acid. Although menthiafolic acid is acyclic and oleuropeic acid cyclic, the acids share similarities such as the presence of an α,β-unsaturated carbonyl group. Moreover, both monoterpene acids can occur as monoesters or diesters of glucose, or more commonly as monoesters of glucose with shared O -linked phenolic moieties such as gallic acid, noreugenin, chromenone, quercetin, or kaempferol. The combination of the α,β-unsaturated carbonyl group in both the monoterpene acid and phenolic moieties, together with the phenolic hydroxyls, gives the compounds particular biological activities with potential commercial applications. In particular, the presence of these functional groups produces a number of important therapeutic properties such as tumor inhibition, carcinogenesis suppression, and antioxidant and antiinflammatory activities. In this chapter, we review all of the glucose esters containing these monoterpenoids identified to date and describe their biological activities and commercial potential as therapeutics. We also discuss how such properties may relate to functional roles in plants such as biotic and abiotic stress responses. Finally, we collate the limited information available on their biosynthesis.