Galactono-1,5-lactone structure

It is of interest to note, however, that the configuration of the hydroxyl on C-3 affects the ease with which the substance is transformed into ascorbic acid in the plant the enzyme concerned reacts more readily with the lactone which has the n-configuration i.e., with the L-galactono ring structure), but in animals the difference is not so obvious. When an unnatural lactone is provided, this differing specificity of the plant and animal enzymes is more clearly shown. With plants (cress seedlings, mung beans, and peas) n-altrono-y-lactone only is converted to n-arabo-ascorbic acid, whereas with rats only D-mannono-y-lactone yields this ascorbic acid. 

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