Metabolism of xylose by the lens. Calf lens in vitro

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Abstract
Xylose is not phosphorylated by the calf lens; it does not interfere with the phosphorylation of glucose by lens dispersions. The formation of lactic acid from glucose is inhibited by xylose in the intact lens but not in lens dispersions. Penetration of glucose into the intact lens is inhibited by xylose. Xylose is oxidized by the calf lens (intact or dispersed) to xylonic acid by means of a dehydrogenase using diphospho-pyridine nucleotide (DPN). Reduction of DPN by dialyzed lens extract at pH 8 is faster with xylose (0.02 [image]) as substrate than with glucose at the same concentration. Comparison between intact lenses incubated in a medium containing glucose, without or with the addition of xylose, shows that the formation of xylonic acid from xylose is accompanied by an increase in the amount of [alpha]-glycerophosphate ([alpha]GP) in the lens and a corresponding decrease in the acid-labile phosphate contained in the nucleotide di- and tri-phosphates. There is no change in ortho-phosphate or in the sum of phosphorylcholine, phosphorylethanolamine, glycerophosphorylcholine and glycerophosphorylethanolamine. It is suggested that, in the presence of xylose, DPN normally available for oxidation of 3-glyceraldehyde phosphate in the glycolytic cycle is preferentially reduced by xylose dehydrogenase to xylonic acid. The observed increase in GP is explained on the assumption that the reduced DPN thus formed is reoxidized, not as in the normal lens through pyruvic acid and lactic dehydrogenase, but by the action of [alpha]GP de-hydrogenase on dihydroxyacetone phosphate.