The conversion of C14-labeled glucose cycloacetoacetate to L-ascorbic acid in germinating Phaseolus radiatus.

The investigations by Xath, Chitale, and Belavady (1) showed that when n-glucose1 and an equimolar amount of sodium acetoacetate is fed to germinating mung beans or injected into rats (a), the production of n-ascorbic acid is greater than with glucose alone. It was further observed (2) that a condensation product of glucose and acetoacetate (glucose cycloacetoacetate ethyl ester) can be easily utilized by germinating mung beans for still greater yield of ascorbic acid, which indicates the possibility that glucose cycloacetoacetate ethyl ester acts as a precursor of ascorbic acid in plant and animals. These findings were later confirmed in this laboratory by paper chromatography (3) and by biological studies on scorbutic guinea pigs (4). Our findings were also confirmed by Thangamani and Sarma (5) with labeled glucose cycloacetoacetate ethyl ester, which gave rise to labeled ascorbic acid. Glucose cycloacetoacetate ethyl ester was first prepared by West (6), and the structure determined by Gonzales (7) was 2-tetrahydroxybutyl-5-methyl-4-carboxyethylfuran. Jackel et al. (8) were the first to show that uniformly labeled glucose, when fed to Chloretonized rats, gives rise to uniformly labeled ascorbic acid. The work of Horowitz, Doerschuk, and King, (9, 10) supported the contention that in animals glucose is directly converted to ascorbic acid without fragmentation but with inversion of the carbon chain. An independent study with radioactive n-glucose-l-Cl4 has been carried out by Loewus, Jang, and Seegmiller (11) in plants. In their experiments with strawberries, glucose was converted to ascorbic acid, maintaining the carbon skeleton intact, but in contrast with the situation in rats, there was no inversion of the molecule. Knowledge concerning the mechanism of formation of ascorbic acid from glucose cycloacetoacetate ethyl ester is still limited. The present investigation was therefore undertaken by labeling glucose cycloacetoacetate ethyl ester at carbon atoms 1 or 6 of the glucose moiety, administering it to mung seeds (Phaseolus radiatus), and determining the distribution of Cl4 in the resulting ascorbic acid. Further observations were made by comparing the yields of labeled ascorbic acid when uniformly labeled glucose or labeled glucose cycloacetoacetate ethyl ester, with the glucose moiety uniformly labeled, was administered to Phaseolus racliatus.