Part of the xanthophyll (or violaxanthin) cycle for controlling the concentration of zeaxanthin in chloroplasts. Catalyzes the two-step mono de-epoxidation reaction. Stereospecific for all-trans xanthophylls. Zeaxanthin induces the dissipation of excitation energy in the chlorophyll of the light-harvesting protein complex of photosystem II. all-trans-violaxanthin + 2 L-ascorbate = all-trans-zeaxanthin + 2 H2O + 2 L-dehydroascorbate all-trans-violaxanthin + L-ascorbate = all-trans-antheraxanthin + H2O + L-dehydroascorbate all-trans-antheraxanthin + L-ascorbate = all-trans-zeaxanthin + H2O + L-dehydroascorbate Inhibited by DTT. reaction significantly slower in the absence of monogalactosyldiacylglyceride. Optimum pH is 5.0. Low constitutive expression in etiolated seedlings and immature yellow leaves. Higher expression in mature green leaves. Associates specifically at acidic pH with monogalactosyldiacylglyceride (MGDG), the major lipid of thylakoids. Belongs to the calycin superfamily. Lipocalin family.