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Xylone isomerase (D-xylose ketol-isomerase, EC 5,3,1,5) have been demonstrated in the cell-free extracts of Stroptomuces canus and Streptomuces malachiticus grown in the presence of xylose. Xylose, glucose and ribose served as substrates for the enzymes of the two strains with respective K_m values of 22, 130, 290 mM (S. canus) and 7,83,637 mM(S.malachiticus), and V_max values of 1,000, 0.087, 0.0222umoles/min/mg protein (S. canus) and 0.312, 0.083, 0.500umoles/min/mg protein (S. malachiticus). L-Rhammose was also isomerized by the crude enzyme solutions of the two strains. The maximal activities of the two xylose-isomerases were observed at pH 7.5 and 75℃. The xylose isomerase activities of the two strains were activated two-three times by Mg^++ or Co^++ as that of control, partially activated by Ba^++ and inhibited by Ni^++, Ca^++ and Zn^++. Particulary, the addition of Mn^++ stimulated xylose-isomerizing activities, but inhibited glucose-isomerizing activities in both strains. However, Cu^++ inhibited xylose-isomerizing activities, while stimulated glucose-isomerizing activities of the enzymes.
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