α-Carotene and its derivatives have a sole chirality in phototrophic organisms ? *

Carotenoids in eukaryotic phototrophic organisms can be classified into two groups; β-carotene and its derivatives, and α-carotene and its derivatives. We reexamined distribution of α-carotene and its derivatives among various taxa of aquatic algae (17 classes) and land plants. α-carotene and its derivatives were found from Rhodophyceae (macrophytic type), Cryptophyceae, Euglenophyceae, Chlorarachniophyceae, Prasinophyceae, Chlorophyceae, Ulvophyceae, Charophyceae, and land plants, while they could not be detected from Glaucophyceae, Rhodophyceae (unicellular type), Chryosophyceae, Raphidophyceae, Bacillariophyceae, Phaeophyceae, Xanthophyceae, Eustigmatophyceae, Haptophyceae, and Dinophyceae. We also analyzed the chirality of α-carotene and/or its derivatives, such as lutein and siphonaxanthin, and found all of them had only (6'R)type, not (6'S)-type.

In this study, to confirm the reliability of chirality, we re-examined distribution of α-carotene and its derivatives among algae, and analyzed their C-6′ chirality using circular dichroism (CD) or nuclear magnetic resonance spectra after purification of the carotenoids.

MATERIALS AND METHODS
Most of microalgae were cultured in the laboratory.Macrophytic seaweeds (Rhodophyceae, Phaeophyceae, Ulvophyceae) were collected at rocky seashore in Awajiisland, Japan.Nearly 40 species were analyzed.

Chirality of α-carotene and/or its derivatives
We analyzed chirality of α-carotene and/or its derivatives from more than 30 species described above including our previous papers (Yoshii et al. 2002;Kusaba et al., 2009), and we obtained their chirality from around 20 species from reliable references.All of them had only (6′R)-type (Fig. 2).

DISCUSSION
In this study, we found that α-carotene and/or its derivatives (Fig. 1) were presented in the phylogenetically limited groups.One exception was Rhodophyceae (red algae), in which macrophytic type contained these carotenoids but unicellular type did not.All of α-carotene and its derivatives examined were (6′R)type, and (6′S)-type was not found (Fig. 2).
In biosynthesis of α-carotene in land plants, both lycopene β-cyclase and lycopene ε-cyclase are needed to produce α-carotene from lycopene.They have high homology with each other, and therefore lycopene ε-cyclase gene might be produced by duplication of lycopene β-cyclase gene (Cunningham et al., 1996).In enzymatic reaction of cyclization, the mechanisms of lycopene β-cyclase, lycopene (6′R)-ε-cyclase, and lycopene (6′S)-ε-cyclase are almost the same; the products are depending on the carbon number to eliminate H + and on the direction of elimination.Therefore, both lycopene ε-cyclases could be exist, but only lycopene  (6′R)-ε-cyclase was found based on the presence of only (6′R)-type.Since the stereochemistry of (6′R)and (6′S)-α-carotenes are different for the direction of ε-end groups (Fig. 2), the binding site on the protein should not be identical.Consequently, the protein moiety might restrict to one chirality of α-carotene, (6′R)-α-carotene.