Biological synthesis of l-ascorbic acid: the conversion of l-galactono-γ-lactone into l-ascorbic acid by plant mitochondria

Abstract

The enzymic conversion of L-galactono-[gamma]-lactone to L-ascorbic acid by mitochondria from pea and mung bean seeds was demon-strated. Neither L-gulono-[gamma]-lactone nor D-mannono-[gamma]-lactone yield ascorbic acid in these circumstances. Galactonic acid is not converted to L-ascorbic acid by these mitochondrial preparations, and the concurrent conversion of L-galactono-[gamma]-lactone to galactonic acid during the reaction accounts for our failure to obtain quantitative yields of ascorbic acid from the lac-tone. The conversion appears to be dependent on the integrity of the mitochondria. The most active preparations have been obtained by extraction with sucrose (0.4[image]) and phosphate (0.1[image]). Arsenate can replace phosphate both in the extraction of the mitochondria and in the reaction medium. The reaction is dependent on the presence of O2, but this may be reduced to a tension of 0.5% without any effect on the reaction velocity. Expts. with cyanide, azide and carbon monoxide suggest that the cytochrome system is involved. 2:4-Dinitrophenol in high concns. does not, but adenosine triphosphate (1 x 10-3 [image]) does inhibit the reaction. There is no evidence to suggest the necessity for the participation of compounds containing high-energy phosphate bonds in the formation of ascorbic acid from galactono-[gamma]-lactone. The enzymic activity of mitochondria prepared from peas increased rapidly in the early stages of germination.