Molecular oxygen and the state of geometric isomerism of intermediates are essential in the carotene desaturation and cyclization reactions in daffodil chromoplasts.

The membrane-bound carotenogenic reaction sequence in daffodil chromoplasts can be subdivided in vitro into three reaction segments by varying the incubation parameters O2 and light. In the first segment, 15-cis-phytoene is desaturated to 15-cis-zeta-carotene (trans elimination of hydrogen) in the dark and in the presence of O2 as an essential cofactor. A photoisomerization of the 15-cis double bond of the accumulated zeta-carotene to trans is the prerequisite for the function of the second segment, the desaturation to 7,9,9',7'-tetra-cis-lycopene (prolycopene, cis elimination of hydrogen). The role of O2 as an electron acceptor is discussed and evidence for an oxidoreductase acting as a redox mediator between the desaturase (forming the polyene chromophore) and O2 is presented. A certain analogy to the desaturation of stearoyl-carrier protein, which also occurs in plastids, is proposed. The third segment, the cyclization of prolycopene, is active only in the absence of O2 and involves additional cis-trans isomerization reactions.