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

Abstract

Xylose isomerase is found in a number of plants. It has been partially purified from wheat germ. The enzyme activity depends on free sulfhydryl groups and on divalent cations, among which Mn++ is the most effective. The Km values for xylose and xylulose are 0.006 [image] and 0.0004 [image] respectively, the relative Vmax, 0.01 and 0.004, respectively at 30[degree]. The equilibrium constants for the isomerization of xylose to xylulose at 0[degree], 30[degree] and 37[degree] are 0.11, 0.22, and 0.24, respectively. At 30[degree], [DELTA]F[degree] is 900 calories/mole and the change in entropy, 11 entropy units/mole. Both the crude and purified preparations show ribose isomerase activity. Mixed substrate studies support the idea that ribose and xylose isomerase activities reside in the same moiety.