Partial Purification & Properties of Xylose & Ribose Isomerase in Higher Plants

The conversion of L-arabinose into the pentose moieties of plant polysaccharides proceeds with little rearrangement of the carbon skeleton (14,18). On the other hand D-ribose and D-xylose suffer considerable rearrangement. Experiments with specifically labeled sugars suggest that these two pentoses must pass through a hexose and indeed through the pentose phosphate pathway (1, 3, 8,10, 14, 17, 18). The discovery by Hassid and his co-workers of Larabinose kinase and the appropriate pyrophosphorylases and sugar nucleotide epimerases readily accounts for the observations with L-arabinose (19). However kinases for D-ribose and D-xylose have not been found in higher plants. A previous communication from this laboratory noted the presence in plant tissues of an enzyme, xylose isomerase which catalyzed the interconversion of D-xylose and D-xylulose (20). The action of a suitable kinase (for which we have presented indirect evidence & which is currently the object of study in our laboratory) could yield D-xylulose-5phosphate which would serve to introduce the xylulose into the metabolic stream of the plant via the pentose phosphate pathway. It is the purpose of this paper to describe a partial purification of xylose isomerase from wheat germ, and to note some of its properties, including its ability to catalyze the interconversion of D-ribose and D-ribulose. The demonstration of xylose isomerase activities in several other plant species point to a possibly general significance of this enzyme.