An L-arabinose isomerase has been purified 12-fold from L-arabinose-induced cultures of Escherichia coli B/r strain F’ araB-24/araB-24. The enzyme is homogeneous in the ultracentrifuge and better than 98% pure by disc acrylamide electrophoresis. The s;,,,~ is 12.65 X lo-l3 set at pH 7.6, 12.72 X lo-l3 set at pH 6.0, and 12.66 X lo-l3 set at pH 5.0. There is no evidence of concentration-dependent dissociation of the enzyme from pH 5.0 to pH 7.6. The molecular weight determined by sedimentation equilibrium is 3.62 f 0.16 X 105. The enzyme exhibits only slight activity toward D-fUCOSe, L-fucose, and D-xylose and no activity toward any of the other compounds tested. Several polyalcohols, from 3 to 5 carbon atoms in length, are effective inhibitors of the enzyme, and at least two of these, L-arabitol and ribitol, are competitive inhibitors. Activation by Mn+f has been demonstrated by means of enzyme dialyzed against ethylenediaminetetraacetic acid, but not with nondialyzed enzyme. An amino acid analysis is presented.
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