d-Xylulokinase and d-Ribulokinase in Higher Plants

Although D-xylose and D-ribose can be utilized in respiration (3, 7,27) or in polysaccharide formation by higher plants, the initial metabolic reactions by which they are introduced into the general metabolic stream have not been firmly established. Several workers (2, 3, 14, 17) have suggested that these aldopentoses are converted to intermediates of the pentose phosphate pathway, probably to D-xylulose 5-phosphate and D-ribulose 5-phosphate, respectively. Previous work in this laboratory (19, 20) has demonstrated the presence of xylose isomerase in plants. A partially-purified preparation of the enzyme catalyzes the isomerization of D-xylose and D-ribose to the corresponding 2-pentuloses. Phosphorylation of D-xylulose or D-ribulose, catalyzed by the appropriate kinase, would complete the sequence of reactions necessary for the introduction of D-xylose or D-ribose into the pentose phosphate pathway in higher plants. This paper presents evidence that higher plants contain D-xylulokinase. The enzyme has been obtained substantially free of xylose isomerase. Botlh D-xylulose and D-ribulose can be phosphorylated by an enzyme preparation from pea meal.