Formation of L-ascorbic acid and oxalic acid from D-glucosone in Lemna minor

The conversion of D-[1-C-14]glucosone to L-ascorbate and oxalate occurs in Lemna minor L. to the same extent as reported earlier in spinach and Pelargonium. A significant amount of L-[1-C-14]ascorbate is converted to oxalate, but virtually none to either glycine or serine. On the other hand, little [2-C-14]glycolate is converted to oxalate but a significant amount to glycine and serine. A time course study on Lemna pulse-labelled with D-[1-C-14]glucosone revealed that the decrease in labelled L-ascorbate is nearly equivalent to the increase in oxalate. These results strongly suggest that L-ascorbate, which has been formed from D-glucosone in Lemna, is converted to oxalate without passing through glycolate and/or glyoxylate as intermediates. Chlorpromazine, a calmodulin antagonist which is known to be an inhibitor of electron transport in photosystem II of spinach chloroplast, markedly lowered the content of L-ascorbate in Lemna possibly due to stimulation of L-ascorbate breakdown.