Ascorbate dioxygenase

Flavonol synthase (FLS E.C.l.14.11.23) catalyzes the committed step in the production of fiavonols by introduction of a double bond between C2 and C3 of the corresponding dihydroflavonols. Like E3H, ELS has been described as a 2-oxoglutatarate-dependent dioxygenase based on its cofactor requirements for 2-oxoglutarate, Fe, and ascorbate. FLS was initially identified in enzyme preparations from illuminated parsley cell suspension cultures [67]. Subsequently, FLS was characterized from the flower buds of Matthiola incana and carnation (Dianthus caryophyllus L.), and it was suggested that there was regulation between flavonol and anthocyanidin biosynthesis [83, 84]. 

Procollagen proline 4-dioxygenase [Fe, ascorbate]— proline hydroxylase ... 

This enzyme [EC 1.14.11.1], also known as y-butyrobe-taine, 2-ketoglutarate dioxygenase, catalyzes the reaction of 4-trimethylammoniobutanoate with S-keto-glutarate (or, 2-oxoglutarate) and dioxygen to yield 3-hydroxy-4-trimethylammoniobutanoate, succinate, and carbon dioxide. Both iron ions and ascorbate are needed as cofactors. 

Livers from mice were manually sectioned into 1 to 1.5 mm cubes. In contrast to liver homogenates or purified enzyme, liver slices did not require exogenous methemoglobin or ascorbic acid for activation of tryptophan dioxygenase. 

In vitro, ascorbate and Fe + ions are frequently used as a source of superoxide for such enzymes as indoleamine dioxygenase. Although ascorbate does have prooxidant and superoxide generating activity (Section 13.3.7), there is no evidence that it is the physiological source of this radical for superoxideutilizing enzymes. 

Studies (40) on indolylamine 2,3-dioxygenase suggested molecular oxygen reduction to superoxide radical by ascorbate. Superoxide radicals have also been implicated in ascorbate-simulated oxygen uptake by isolated chloroplasts (41,42). 

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