Effect of inhibitors on the formation of stereoisomers in the biosynthesis of beta carotene in dunaliella bardawil
Shaish, A.; Avron, M.; Ben Amotz, A.
Plant and Cell Physiology 31(5): 689-696
ISSN/ISBN: 0032-0781 Accession: 007250716
An assay system was developed in which the effect of inhibitors of .beta.-carotene biosynthesis in Dunaliella bardawil could be tested. Since D. bardawil can be induced to accumulate over 10% of its dry weight as .beta.-carotene, it is particularly suitable for such studies. Norflurazon a desaturation inhibitor, caused the accumulation of phytoene, or of phytoene and phytofluene, depending on the concentration employed. J-334, a substituted 6-methylpyrimidine which also inhibits desaturation, caused the accumulation of .beta.-zeacarotene, .zeta.-carotene and phytoene in different proportions, depending on the concentration employed. The cyclization inhibitors, nicotine, CPTA and MPTA, severely affected the growth and survival of the alga, and their effects could therefore not be studied directly. However, their action was observed indirectly by following the transformation of phytoene in norflurazon-pretreated phytoene-rich algae. Under these conditions, presence of the cyclase inhibitors caused the transformation of phytoene to lycopene, rather than to .beta.-carotene. The accumulated .beta.-carotene or the intermediates .beta.-zeacarotene, lycopene, .zeta.-carotene, phytofluene and phytoene in D. bardawil were all composed of two stereoisomers, tentatively assigned as the all-trans stereoisomer (55%) and the 9-cis stereoisomer (45%). This suggests that the isomerization reaction which leads to the production of the presumed 9-cis isomers occurs early in the pathway of carotene biosynthesis, at or before phytoene, with no isomerization during the further transformations leading to .beta.-carotene.