d-Glucosone and l-Sorbosone, Putative Intermediates of l-Ascorbic Acid Biosynthesis in Detached Bean and Spinach Leaves

d-[6-(14)C]Glucosone that had been prepared enzymically from d-[6-(14)C]glucose was used to compare relative efficiencies of these two sugars for l-ascorbic acid (AA) biosynthesis in detached bean (Phaseolus vulgaris L., cv California small white) apices and 4-week-old spinach (Spinacia oleracea L., cv Giant Noble) leaves. At tracer concentration, (14)C from glucosone was utilized by spinach leaves for AA biosynthesis much more effectively than glucose. Carbon-14 from [6-(14)C]glucose underwent considerable redistribution during AA formation, whereas (14)C from [6-(14)C]glucosone remained almost totally in carbon 6 of AA. In other experiments with spinach leaves, l-[U-(14)C]sorbosone was found to be equivalent to [6-(14)C]glucose as a source of (14)C for AA. In the presence of 0.1% d-glucosone, conversion of [6-(14)C] glucose into labeled AA was greatly repressed. In a comparable experiment with l-sorbosone replacing d-glucosone, the effect was much less. The experiments described here give substance to the proposal that d-glucosone and l-sorbosone are putative intermediates in the conversion of d-glucose to AA in higher plants.