Spectroscopic, ligand binding, and enzymatic properties of the spleen green hemeprotein. A comparison with myeloperoxidase.

The bovine spleen green hemeprotein, a peroxidase which exhibits spectrophotometric properties similar to those of granulocyte myeloperoxidase, was purified using an improved method. The ligand affinity of the ferric enzyme was spectroscopically determined using chloride and cyanide as exogenous ligands. The pH dependence of the apparent dissociation constant of the enzyme-chloride complex showed the presence of a proton dissociable group with a pKa value of 4 on the enzyme; chloride binds to the enzyme when this group is protonated with a dissociation constant of 60 microM. The cyanide affinity of the enzyme is also regulated by the group with a pKa value of 4, but in this case cyanide binds to the unprotonated enzyme with a dissociation constant of 0.6 microM; only the protonated, uncharged form of cyanide reacts with the enzyme. Cyanide binding was competitively inhibited by chloride, and chloride binding was also competitively inhibited by cyanide. The EPR spectrum of the resting enzyme exhibited a rhombic high spin signal at g = 6.65, 5.28, and 1.97 with a low spin signal at g = 2.55, 2.32, and 1.82. Upon formation of the chloride complex, the spectrum was replaced with a new high spin EPR signal with g-values of 6.81, 5.04, and 1.95. The cyanide complex showed a low spin EPR signal with g-values of 2.83, 2.25, and 1.66. Examination of the enzymatic activity of the spleen green hemeprotein by following the chlorination of monochlorodimedon has indicated that the enzyme has the same chlorinating activity as myeloperoxidase; the spleen green peroxidase can catalyze the formation of hypochlorous acid from hydrogen peroxide and chloride ion. Comparison of the present data with those of myeloperoxidase has led to the conclusion that the structure of the iron center and its vicinity in spleen green hemeprotein is very similar, if not identical, to that of myeloperoxidase. The spleen enzyme can thus be used as a model to study the active center, and its environment, in myeloperoxidase.