Showing papers by "Frank A. Loewus published in 1990"

Conversion of l-Sorbosone to l-Ascorbic Acid by a NADP-Dependent Dehydrogenase in Bean and Spinach Leaf.

Abstract: An NADP-dependent dehydrogenase catalyzing the conversion of l-sorbosone to l-ascorbic acid has been isolated from Phaseolus vulgaris L. and Spinacia oleracea L. and partially purified. It is stable at −20°C for up to 8 months. Molecular masses, as determined by gel filtration, were 21 and 29 kilodaltons for bean and spinach enzymes, respectively. Km for sorbosone were 12 ± 2 and 18 ± 2 millimolar and for NADP+, 0.14 ± 0.05 and 1.2 ± 0.5 millimolar, for bean and spinach, respectively. Lycorine, a purported inhibitor of l-ascorbic acid biosynthesis, had no effect on the reaction.

D-Glucosone and L-sorbosone, putative intermediates of L-ascorbic acid biosynthesis in detached bean and spinach leaves.

Abstract: d-[6-14C]Glucosone that had been prepared enzymically from d-[6-14C]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, 14C from glucosone was utilized by spinach leaves for AA biosynthesis much more effectively than glucose. Carbon-14 from [6-14C]glucose underwent considerable redistribution during AA formation, whereas 14C from [6-14C]glucosone remained almost totally in carbon 6 of AA. In other experiments with spinach leaves, l-[U-14C]sorbosone was found to be equivalent to [6-14C]glucose as a source of 14C for AA. In the presence of 0.1% d-glucosone, conversion of [6-14C] 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.