Carotenoids of the Fruiting Gliding Myxobacterium, Myxococcus sp. MY-18, Isolated from Lake Sediment: Accumulation of Phytoene and Keto-myxocoxanthin Glucoside Ester.

The precise chemical structures of two carotenoid glucoside esters, keto-myxocoxanthin glucoside ester isolated from Myxococcus sp. MY-18 and dehydrorhodopin glucoside ester from cells cultured in a nicotinecontaining medium, were determined as 1'-[(6-O-acyl-β-D-glucopyranosyl)oxy]-3',4'-didehydro-l',2'-dihydro-β,ψ-caroten-4-one and 1-[(6-O-acyl-β-D-glucopyranosyl)oxy]-3,4-didehydro-1,2-dihydro-ψ,ψ-carotene, respectively. They had β-D-glucoside, whose C-6 hydroxyl group was esterified. The major fatty acid was 11-methyldodecanoate with minor components of n-dodecanoate, n-tetradecanoate, and 13-methyltetradecanoate. This composition is different from that of cellular lipids, and the unusual accumulation of phytoene (more than 70% of total carotenoids) is characteristic of this strain. The possible biosynthetic pathway for the carotenoid glucoside ester starts with lycopene being cyclized to γ-carotene, from which myxocoxanthin is produced by hydration and desaturation. β-D-Glucose was attached to the carotenoid, and the fatty acid ester was next. The pathway for the introduction of a keto group to the carotenoid moiety was not clear, however. The final product was keto-myxocoxanthin glucoside ester. Cyclization was completely inhibited by 6.0mM nicotine in the medium, in which case, the final product was dehydrorhodopin glucoside ester.