Biosynthesis of L-ascorbic acid from myo-inositol

Abstract

L-Ascorbic acid biosynthesis from myo-inositol was studied in hen-kidney and rat-liver slices and homogenates. For comparison, D-glucuronic acid, D-glucurono-6,3-lactone, and L-gulono-1,4-lactone were used as substrates, in order to clarify the metabolic significance of the myo-inositol pathway in L-ascorbic acid biosynthesis. A paper-chromatographic method was developed for the separation of labelled L-ascorbic acid from the substrate, myo-inositol, and other glucuronic acid pathway metabolites. The glucuronolactonase activity was determined by recording the pH changes due to the liberation of protons during lactone hydrolysis. Hen-kidney slices were able to synthesize L-ascorbic acid from myo-inositol as from D-glucuronic acid, D-glucurono-6,3-lactone, and L-gulono-1,4-lactone. The rate of L-ascorbic acid synthesis from myo-inositol was so high that this pathway is biologically significant. Rat-liver slices and homogenates were unable to use myo-inositol as a substrate due to the lack of myo-inositol oxygenase, whereas other metabolites were converted into L-ascorbic acid.