Using an in vitro system to study lipid dependence of the xanthophyll cycle de-epoxidase activity, such as violaxanthin de-epoxidase (VDE) and diadinoxanthin de-epoxidase (DDE), we found that replacement of monogalactosyldiacylglycerol (MGDG) with digalactosyldiacylglycerol (DGDG) or phosphatidylcholine (PC) in the assay medium resulted in a strong inhibition of xanthophyll cycle pigments, violaxanthin (Vx) and diadinoxanthin (Dx), de-epoxidation. On the other hand, replacement of MGDG with phosphatidylethanolamine (PE) sustained a high activity of these enzymes, in spite of the very different chemical character of these two lipids. We have also found that Ddx and Vx solubility in lipids depends on the lipid species used. Using phosphorus NMR measurements, we detected the existence of the inverted hexagonal pase (H_II) in a binary (MGDG/PC) lipid mixture as well as in thylakoid membranes. The obtained results clearly indicate that only H_II forming lipids (MGDG and PE) may effectively support Vx or Ddx de-epoxidation, whereas bilayer-forming lipids (DGDG and PC) are not effective in this process. Hence, a conclusion is drawn that VDE and DDE activity depends not on the chemical character of lipids but on the kind of structure they form in water environment. VDE and DDE constitute a new family of enzymes whose activity depends on the presence of inverted hexagonal structures of lipids. We propose a molecular model of Vx and Dx de-epoxidation in which H_II forming lipids play a crucial role.

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