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 C₁₃-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 C₁₃-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 C₁₃-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 ¹³C markers from carotenoids to norisoprenoids in berries between véraison and berry maturity. All of these findings are major arguments in favour of the hypothesis that glycosylated C₁₃-norisoprenoids derive from carotenoids in grape berries. Carotenoids are mostly synthesised from the first stage of fruit formation until véraison, and then degrade between véraison and maturity to produce glycosylated C₁₃-norisoprenoids and other compounds.