myo-inositol oxygenase from rat kidneys. Substrate-dependent oligomerization

myo-Inositol from rat kidneys, an oligomeric protein with apparent molecular mass of about 270 kDa can be dissociated under mild conditions to structured 16.8-kDa monomers. This dissociation can be reversed at high protein concentrations at room temperature. The corresponding apparent dimerization constant K2app = 1.38 x 10(5) M-1, the corresponding rate constant k2 = 350 s-1.M-1, and the apparent constant for the association of dimers, K4app = 2.7 x 10(6) M-1. Reassociation is significantly enhanced in the presence of the substrate and iron(II) (K2app = 9.8 x 10(5) M-1; K4app = 3.75 x 10(6) M-1, k2 = 1750 s-1.M-1, at 20 mM myo-inositol and 0.5 mM FeSO4). Under these conditions almost 100% of the original enzymatic activity was reconstituted. Monomers, with or without bound ligands, lack catalytic activity, whereas the dimer is likely to be the elementary active enzyme-building unit. The effects of myo-inositol on the dimerization lead to the conclusion that this step is both mediated and facilitated by the substrate.