METABOLISM OF MYO-INOSITOL IN PLANTS: CONVERSION TO PECTIN, HEMICELLULOSE, D-XYLOSE, AND SUGAR ACIDS

Abstract

myo-Inositol is a ubiquitous constituent of higher plants, both in the free state and as the hexaphosphate, phytic acid;I-3 yet its metabolism in plant tissue has thus far remained obscure. myo-Inositol is known to be converted to D-glucuronic acid by an oxidase in rat kidney4 and in a yeast,5 and preliminary evidence suggests that a similar reaction occurs in barley roots.6 D-Glucuronic acid and its lactone have been shown to be directly converted by detached plant tissues to i-gulonic acid and to the D-galacturonosyl residues of pectin, and, by decarboxylation, to free D-xylOse and to the D-xylosyl and-arabinosyl residues of hemicellulose.7-10 If myo-inositol is metabolized via D-glucuronic acid, one would expect it to be converted to the same end products. Evidence for such a conversion of myo-inositol to D-glucuronic acid, D-Xylose, L-gulonic acid, pectin, hemicellulose, and related metabolites is presented in this report. Materials and Methods.-myo-Inositol-2-H3 was prepared from myo-inososell by reduction with NaBH312. The resulting mixture of myo-and scyllo-inositol was converted to hexa-O-acetyl derivatives which could be separated by means of their differential solubilities in ethanol. Hydrolysis of hexa-O-acetyl myo-inositol with Ba(OH)2 in methanolic solution'3 yielded free myo-inositol, which was re-crystallized from water and ethanol. myo-Inositol-2-C'4 was generously provided by Laurens Anderson.'4 The labeled compounds were administered in aqueous solution through the cut stems of parsley (Petroselinum) leaves or strawberry (Fragaria) fruits. Pertinent details are listed for each experiment in Table 1. After the indicated period of metabolism , cellular constituents were separated as previously described.7 Radioactive determinations were made in a napthalene-dioxane-diglyme system'5 using a liquid scintillation counter with an efficiency of 14% for H3 and 49% for C14. Results.-The distribution of label in various fractions at the end of each experiment is listed in Table 1. From 35 to 56 per cent of the administered label remained in the insoluble residue after repeated washes with 70 per cent ethanol. Hydrolysis of these residues revealed that a considerable portion of the activity (as much as 24% of that administered in experiment 2) was in the D-galacturonosyl residues of