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

In Narcissus chromoplast membranes, phytoene is converted into coloure d carotenes by two desaturases which differ in stereospecificity and w hich can be distinguished by herbicidal compounds. The desaturases rep resent flavoproteins, which gain FAD in an assisted folding pathway af ter being imported into the organelle. Electrons liberated in the desa turation 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 whi ch is membrane-peripherally localized. The end product of the desatura tion, 7,9,7',9,-tetra-cis-lycopene (prolycopene), is the preferred sub strate for cyclization, which is accompanied by an NADPH-dependent iso merization yielding trans cyclic products.