Photoisomerization of zeta-carotene stereoisomers in cells of Euglena gracilis mutant W3BUL and in solution.

Abstract

Abstract— Dark‐grown cells of Euglena gracilis var. bacillaris bleached mutant W3BUL are shown by a novel alumina HPLC method to accumulate a cis isomer of ζ‐carotene (cis I; δmax at 286,296,377,398,422 nm; εmM= 97 at 398 nm). Illumination of cells with saturating blue light converts nearly all of this to the trans isomer (δmax at 379,400,425 nm) and a small amount of a second cis isomer (cis II; δmax at 285.5,296, 374, 394.5, 419 nm; εmM= 111 at 394.5 nm) with no significant changes in any other carotenoids. Photoisomerization of the purified isomers in hexane yields the same mixture of stereoisomers in all three cases, and this mixture is similar to that produced in the cells. Photoisomerization of the purified cis isomers in hexane occurs readily with first order kinetics indicating that no additional photosensitizer or catalyst is necessary for the reaction in vivo. Wild‐type cells grown in darkness in the presence of 72 μM J334 accumulate ζ‐carotene almost exclusively with approximately equal amounts of the cis I and trans isomers, thus cis I is not unique to the mutant. Cis I is identified as 15‐cis‐ζ‐carotene by UV, visible, infrared and mass spectra; cis II may be the 13‐eis isomer. Since W3BUL also accumulates cis isomers of phytoene and phytofluene while the other carotenoids are trans, it is suggested that, in Euglena, ζ‐carotene is the point of isomerization from cis to trans in the biosynthetic pathway.