L-galactono-y-lactone

Studies of the effect of lycorine on the AA biosynthesis are still in progress. In vivo conversion of L-galactono-y-lactone to ascorbate is being carried out in all of the different nine plants investigated. 

Lycorine (1), the most frequent and characteristic of the Amaryllidaceae alkaloids, has been reported to be a powerful inhibitor of ascorbic acid (l-Asc) biosynthesis (436,437), and thus has proved to be a useful tool in studying Asc-dependent metabolic reactions in L-Asc-synthesizing organisms (438,439). Lycorine is actually a powerful inhibitor of the activity of L-galactono-y-lactone dehydrogenase, the terminal enzyme of l-Asc biosynthesis (440-443), which appears to be localized in the mitochondrial membrane (444,445). Galanthine (7) also has a high capacity to inhibit ascorbic acid biosynthesis (437). 

Several species of fungi Lipomyces starkei, Saccharomyces marxianus, Sac-charomyces cerevisiae, and Candida macedoniencis) are reported to synthesize ascorbic acid (Heick et ai, 1972). L-Galactono-y-lactone seems to be the immediate precursor to ascorbic acid, because this lactone is the best substrate for the terminal enzyme of the pathway, L-galactono-y-lactone oxidase (Nishikimi et al., 1978). 

Mapson, L. W., and Breslow, E., 1958, Biological synthesis of L-ascorbic acid L-Galactono-y-lactone dehydrogenase, Biochem. J. 68 395-406. 

D-Galactose o-Galacturonic acid — L-Galactonic acid — L-Galactono-Y-lactone... 

Since the change L-galactono-y-lactone — L-ascorbic acid involves the transfer of hydrogen, the presence of a suitable hydrogen acceptor was essential. Of several tried, oxygen was the only one found to be effective in its absence there was no synthesis. 

L-Galactono-y-lactone L-Gulono-7-lacton L-Arabino-7-lactone ° Erythroascorbic L-Sorboson ... 

Isbell made use of this reaction to S3mthesise vitamin C from L-galactono-y-lactone. The latter was first converted into the 2-0x0 derivative (see p. 83). A commercial synthesis of ascorbic acid is also based on this isomerisation. Bertrand s sortose bacterium, Acetobacter xylinium, converts sorbitol to L-sorbose (XCI) and this on condensation with acetone yields the 2,3 4,5-di-O-isopropyhdene derivative (XCII). Oxidation and hydrolysis of this... 

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