The physico-chemical properties of the purified glucose isomerases [D-xylose ketol iso merase, EC 5.3.1.51 of Streptomyces olivochromogenes and Bacillus stearothermophilus were examined. The molecular size and shape of both enzymes were similar. The molecular weights, sedimentation coefficients, partial specific volumes, diffusion constants and Stokes' radii of the Streptomyces and Bacillus enzymes were determined to be 120,000 and 130,000, 7.55 S and 9.35 S, 0.725 and 0.736 ml/g, 5.87 >: 10-' and 6.82 x 10-7cm2/sec, and 51 and 53 A,

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