Metabolism of linalool and substrate analogs in grape berry mesocarp of Vitis vinifera L. cv. Morio Muscat: demonstration of stereoselective oxygenation and glycosylation

The oxidative metabolism of linalool in Vitis vinifera L. cv. Morio Muscat has been investigated by in vivo feeding experiments using the regioselectively stable isotope-labelled substrates d5-(3R/S)-linalool, d6-(3R)-linalool, d2,18O-(3R/S)-linalool and the substrate analog (3R)-linalyl methyl ether. The enantiomeric and diastereoisomeric ratios of the metabolites were determined by means of enantioselective multidimensional gas chromatography–mass spectrometry. Stereoselective transformation to diendiol II and the furanoid and pyranoid linalool oxides could be demonstrated. Other metabolites like diendiol I, hotrienol and 8-hydroxy linalool were detectable as well. The studies indicate that the corresponding metabolites were efficiently glycoconjugated. Time course studies including the determination of conversion rates revealed that the activity of these secondary transformations is dependent on the ripening stage. The mass spectrometric analysis of the labelled linalool oxides derived from mixed labelled d2,18O-linalool and the stereoselective anaylsis of the metabolite 6-methoxy-2,6-dimethyloct-7-en-2,3-diol, which is derived from the substrate analog (3R)-linalyl methyl ether, give evidence that the furanoid linalool oxides are generated via two different reaction pathways.