Biogeneration of C13-norisoprenoid compounds:: experiments supportive for an apo-carotenoid pathway in grapevines

The first step of the proposed biogenetic pathway in grapes that leads from carotenoids to C-13-norisoprenoids involves the enzymatic degradation of carotenoids by regiospecific oxygenases. Chemical, photochemical and oxidase-coupled degradation of carotenoids to norisoprenoids have been studied in vitro and enzymatic systems shown to be involved in mammals. However, no enzymatic system has been shown to be involved in the formation Of C-13-norisoprenoids in grapes, despite all recent studies carried out on grapevines supporting such a model. These findings include the preponderance of norisoprenoids possessing 13 carbon atoms that thereby indicates the specificity of cleavage, the configuration of the asymmetric centres and axes common to C-13-norisoprenoids and the corresponding carotenoids, the negative correlations observed between the levels of norisoprenoids and carotenoids during berry development, and the in vivo transfer of C-13 markers from carotenoids to norisoprenoids in berries between veraison and berry maturity. All of these findings are major arguments in favour of the hypothesis that glycosylated C-13-norisoprenoids derive from carotenoids in grape berries. Carotenoids are mostly synthesised from the first stage of fruit formation until veraison, and then degrade between veraison and maturity to produce glycosylated C-13-norisoprenoids and other compounds. (C) 2002 Elsevier Science B.V. All rights reserved.