Partially folded conformations of inositol monophosphatase endowed with catalytic activity

The stability of porcine brain inositol monophosphatase in the presence of increasing concentrations of urea was investigated at pH 7.5. Exposure of the enzyme to 8 M urea brings about the dissociation of the dimeric species of 58 kDa into monomeric forms as revealed by gel filtration chromatography. Unfolding of the protein by 8 M urea results in a decrease of the ellipticity at 220 nm (20%) together with a perturbation of the near-UV circular dichroism spectrum. Urea-treated inositol monophosphatase binds Co²⁺ ions with a dissociation constant of 3.3 μM. The enzyme is catalytically competent when assayed with 4-nitrophenyl-phosphate in the presence of the activating ion Co²⁺ at pH 7.5 in 8 M urea. The apparent activation constant for Co²⁺ is 2.5 mM. It is postulated that partially folded conformations of monomeric species preserve their catalytic function because the affinity of Co²⁺ ions for the metal coordination center of the protein is not perturbed by exposure to 8 M urea.