BIOCHEMICAL ASPECTS OF CAROTENE DESATURATION AND CYCLIZATION IN CHROMOPLAST MEMBRANES FROM NARCISSUS-PSEUDONARCISSUS

In Narcissus chromoplast membranes, phytoene is converted into coloured carotenes by two desaturases which differ in stereospecificity and which can be distinguished by herbicidal compounds. The desaturases represent flavoproteins, which gain FAD in an assisted folding pathway after being imported into the organelle. Electrons liberated in the desaturation are transferred to oxygen via an intermediate quinone carrier. The possibility of a co-reduction mechanism is discussed. The redox-state of quinones is regulated by an NADPH: quinone oxidoreductase which is membrane-peripherally localized. The end product of the desaturation, 7,9,7',9,-tetra-cis-lycopene (prolycopene), is the preferred substrate for cyclization, which is accompanied by an NADPH-dependent isomerization yielding trans cyclic products.