Effect of excess cadmium ion on the metal binding site of cabbage histidinol dehydrogenase studied by 113Cd-NMR spectroscopy.

The enzymatic reaction of histidinol dehydrogenase (HDH) was stimulated by about maximally 75% on the addition of Cd2+ ion to the reaction mixture. 113Cd-substituted HDH in the presence of excess Cd2+ has been studied by 113Cd-NMR. 113Cd2+ less than 1 equiv. per subunit preferentially binds to the catalytic metal binding site of the apoenzyme. Further addition of the metal ions causes the structural change of the enzyme including the catalytic metal binding site. HDH takes at least three discernible states, which may correspond to the more or less active forms of the enzyme induced by metal ions.