Synthesis of L(+)tartaric acid from 5-keto-D-gluconic acid in Pelargonium

5-Keto-D-[1-14C]gluconic acid, the most effective precursor of L(+)tartaric acid among all labeled compounds which have ever been tested in grapes, was found to be a good precursor of L(+)tartaric acid in a species of Pelargonium. The synthesis of labeled L(+)tartaric acid from D-[1-14C]glucose in Pelargonium was remarkably depressed when a 0.5% solution of D-gluconate or 5-keto-D-gluconate was administered continuously to leaves together with D-[1-14C]glucose. Our results provide strong evidence that D-[1-14C]glucose is metabolized in Pelargonium to give labeled L(+)tartaric acid via (probably D-gluconic acid and) 5-keto-D-gluconic acid without passing through L-ascorbic acid. Labeled L-idonic acid was found in young leaves of Pelargonium which had been labeled with L-[U-14C]ascorbic acid. The synthesis of the labeled L-idonic acid increased when a 0.1% solution of L-threonate was administered continuously to leaves together with L-[U-14C]ascorbic acid. Specifically labeled compounds, recognized as the members of the synthetic pathway for L(+)tartaric acid from L-ascorbic acid via L-idonic acid in grapes, were administered to young leaves of Pelargonium. Each compound (2-keto-L-[U-14C]idonic acid, L-[U-14C]idonic acid, 5-keto-D-[1-14C]gluconic acid and 5-keto-D-[6-14C]gluconic acid) was partly metabolized, as in grapes. The metabolic pathway starting from L-ascorbic acid to L(+)tartaric acid via L-idonic acid, however, did not actually contribute to the synthesis of L(+)tartaric acid in Pelargonium probably because the activity of each metabolic step was much lower than that observed in grapes.