D-Xylose isomerase (D-xylose ketol isomerase, EC 5. 3. 1. 5) was purified from D-xylose-grown cells of homofermentative Lactobacillus xylosus and compared with D-xylose isomerase of heterofermentative Lactobacillus brevis. The molecular weight was estimated to be 183, 000 by thin layer chromatography on Sephadex G-200. The enzyme was composed of four subunits each having a molecular weight of 45, 000. The optimum pH was 7.5, and the enzyme was more stable at the alkaline pH region than L. brevis enzyme. The former was stable at pH from 6.5 to 11.0, but the latter was stable in a narrow neutral to slightly acidic pH (5.7 to 7.0). Other properties resembled those of L. brevis enzyme. The enzyme was active on D-xylose and D-glucose. Michaelis constant for D-xylose was 5.3mM. The enzyme activity was inhibited competitively by xylitol with an inhibition constant (Ki) of 7mM.

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