Metabolites Involved in Oleuropein Accumulation and Degradation in Fruits of Olea europaea L.: Hojiblanca and Arbequina Varieties

Abstract

The biosynthetic pathway of oleuropein (from 7-ketologanin, oleoside-11-methyl ester, 7-β-1-d-glucopyranosyl-11-methyl oleoside, and ligstroside to oleuropein) was investigated in two fruit species of Oleaceae, namely, Arbequina and Hojiblanca. Main oleuropein precursors and their metabolites, produced by the enzymatic hydrolysis mediated by β-glucosidase, were identified and quantified to establish the oleuropein transformation pathway. Changes in the concentration of these compounds were measured by direct control of in vivo fruit tissue during their ripening. High contents of aglycones at the initial stage of the process were caused by the high activity of β-glucosidase, which supports that oleuropein biosynthesis is coupled with enzymatic hydrolysis, producing its aglycone form. The low oleuropein content at this initial stage was caused by the imbalance between catabolic and anabolic pathways, favoring the former ones. Once the main polyphenol synthesis phase was completed, the biosynthetic capacity diminished and the content of all compounds decreased. Mass balance revealed that precursors of oleuropein, which are rapidly transformed by β-glucosidase and esterases, scarcely contributed to the accumulation of oleuropein. The biosynthetic pathway proposed by Damtoft applies for both varieties, but our study reveals that the β-glucosidase enzyme is involved in oleuropein synthesis. This enzyme shows high substrate specificity to oleuropein, which consequently is degraded to its aglycone form, with diminished efficacy of oleuropein biosynthesis. Different enzymatic activities of varieties will result in oleuropein accumulation and metabolic transformation of phenols.

The Role of Polyphenoloxidase, Peroxidase, and β-Glucosidase in Phenolics Accumulation in Olea europaea L. Fruits under Different Water Regimes

“…The up-regulation of both identified Oeβ-GLU12-like isoforms, showing a high similarity with Oeβ-GLU (involved in oleuropein deglycosylation), and the peak of β-GLU activity further supported their predominant role in the oleuropein catabolic processes occurring during early stages of fruit development and contributing to the composition of olive phenolics ( Figures 3C, 4E,F ). These results agree with those reported by Gutierrez-Rosales et al (2010), who hypothesized that 3,4-DHPEA-EDA was formed via oleuropein by β-GLU activity. Transcription levels of OeGLU12 - like1 and like2 were higher in FI trees, unlike enzyme activity and 3,4-DHPEA-EDA content, which were both higher in RF trees.…”

“…The quantity and localization of in different fruit tissues affect the phenolic composition and oil quality of VOO during extraction (Luaces et al, 2007; Servili et al, 2007a). Endogenous glycosidase and esterase activities were hypothesized to play a pivotal role in the regulation of oleuropein hydrolysis (Amiot et al, 1989; Gutierrez-Rosales et al, 2010, 2012). Recently, several members of the β-glucosidase family have been found in proteomic and transcriptomic studies (Corrado et al, 2012; Bianco et al, 2013).…”

Whole Lyophilized Olives as Sources of Unexpectedly High Amounts of Secoiridoids: The Case of Three Tuscan Cultivars

“…This phenomenon had already been described for the secoiridoids by other authors 15,16 and was in agreement with a previous study on these cultivars. 18 As shown in Figure 4, a similar trend was seen for oleuropein and ligstroside over time in freeze-dried Moraiolo olives.…”

“…It can be hypothesized that the decrease of oleuropein is due to the rupture of the ester bond ( Figure 5) mediated by the esterase enzyme, with hydroxytyrosol and oleoside 11-methyl ester formed by this rupture. 15 The results from the other two samples confirmed this hypothesis, because the low variations of oleuropein were related to a nonsimultaneous increase in hydroxytyrosol and oleoside 11-methyl ester.…”

“…Other research groups 15,16 have also studied the phenolic behavior in freeze-dried olives. Nevertheless, in this case the freeze-drying process was conducted only after the olives were broken into smaller pieces to allow the enzymes to come immediately into contact with the olive's water and phenolic compounds.…”