Conversion of L-Ascorbic Acid to L-Idonic Acid, L-Idono-γ-lactone ane 2-Keto-L-idonic Acid in Slices of Immature Grapes

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Abstract
L-[1-14C] Ascorbic acid ([1-14C]AA) was vacuum-infiltrated into slices of immature grapes (Vitis labrusca L., cv. “Delaware”) after which incorporation of 14C into each metabolite was investigated. Under the experimental conditions used, the metabolic reaction of AA proceeded at a constant rate within the reaction period (5 hr). As a result, 14C first appeared in three metabolic products; L-idonic acid, L-idono-γ-lactone and 2-keto-L-idonic acid (=2-keto-L-gulonic acid), after which incorporation of 14C into tartaric acid (TA) took place. When slices of grapes were treated with iodoacetic acid, the incorporation of 14C into TA completely stopped and in the inhibitor experiment, the amount of 14C which had been incorporated into TA in the control was found to be divided among L-idonic acid, L-idono-γ-lactone and 2-keto-L-idonic acid. These results are strong evidence that at least one of these three compounds is the effective precursor of TA and is located on the metabolic pathway between AA and TA in the grape.

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