Homogeneous hog kidney myo-inositol oxygenase (EC 1.13.99.1) has very low catalytic activity unless activated. A survey of the requirements for activation indicates the following: maximum activity is obtained when both 1 n m Fe(II) and 2 l l l~ L-cysteine are present; either reagent alone gives very little activation; at low concentrations, the apparent K, values for activation by Fe(I1) and L-cysteine are 0.27 and 0.37 111~, respectively, but at high concentrations of either reagent, the

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... ty is decreased; no other metal ion can replace the Fe(I1); although L-cysteine is the best activator in the presence of 1 l l l~ Fe(II), high activity is also observed when it is replaced by D-CySteine, DL-penicillamine, or y-L-glutamyl-L-cysteine; activation is not a property of all sulfhydryl compounds, however, because glutathione, 8-mercaptoethanol, and L-cysteine ethyl ester cause very little activation; 4 l l l~ quinolinic acid in the presence of 0.2 to 1.0 l l l~ Fe(II) causes 60 to 70% of the rate given by L-cysteine and its effect is additive to that of r.-cysteine; the quinolinic acid effect is specific because other closely related compounds do not cause activation. With Fe(II) and L-cysteine as activators, the maximum velocity varies with pH and is greatest at pH 6.0, but the apparent K, for inositol is constant at 5 l l l~ from pH 4.5 to 7.0. With Fe(II) and quinolinic acid as activators, the apparent K , for inositol is 0.2 m at pH 6.0. The oxygenase is very specific for myo-inositol as substrate, but some analogs are inhibitors. Other compounds which inhibit the enzyme in varying degrees are sulfhydryl reagents, complexing agents, nucleoside di-and triphosphates, a-ketoacids, and the barbiturates, barbital and phenobarbital. The enzymic activity is abolished by low concentrations of either oxidants or reductants, but is not appreciably affected by catalase or superoxide dismutase. In the preceding paper (l), the purification and physical characterization of two forms of myo-inositol oxygenase (EC 1.13.99.1) from hog kidneys are described. One form of the enzyme has a molecular weight of 65,000, is homogeneous by several criteria, and is maximally active at pH 6.0. In the other form, the oxygenase is part of an enzyme complex which also