Loss of Hydrogen from Carbon 5 of d-Glucose during Conversion of d-[5-H,6-C]Glucose to l-Ascorbic Acid in Pelargonium crispum (L.) L'Hér.

Conversion of d-[5-(3)H,6-(14)C]glucose to l-ascorbic acid in detached apices of Pelargonium crispum (L.) L'Hér cv Prince Rupert (lemon geranium) was accompanied by complete loss of tritium in the product. Chemical degradation of d-glucose which was recovered from the labeled apices yielded d-glyceric acid (corresponding to carbons 4, 5, and 6 of glucose) with a (3)H:(14)C ratio of 4 to be compared with 9, the ratio in d-[5-(3)H,6-(14)C]glucose initially. Conversion of d-[6-(3)H,6-(14)C]glucose in the same tissue was accompanied by retention of tritium in l-ascorbic acid with a (3)H:(14)C ratio comparable to that of compounds from the hexose pool. Results indicate that during l-ascorbic acid biosynthesis from glucose in Pelargonium crispum hydrogen at carbon 5 undergoes exchange with the medium, suggesting an epimerization at this carbon atom.