Fluoride and Formate Interaction with Liver and Erythrocyte Catalases as Revealed by Solvent-Proton Magnetic Relaxation

The temperature dependence of solvent proton magnetic relaxation rates in solutions of bovine liver and human erythrocyte catalases has been determined for the native enzymes and in the presence of formate, fluoride, and formate + fluoride. A significant enhancement of proton magnetic relaxation (¹H NMR) rates occurred on addition of fluoride. From the frequency dependence of ¹H NMR rates, the correlation time for the iron-electron and proton dipoledipole interaction, τc, was calculated together with possible numbers of exchanging protons which closely approach the heme iron and their corresponding nuclear-electron interspin distances. These data were not consistent with the hypothesis that the protons of a water molecule at the sixth coordination position of the iron could account for the observed ¹H NMR rates. In solutions of catalase in the presence of both formate and fluoride, these rates were higher than in catalase-formate. In the former solution electron paramagnetic resonance measurements demonstrated that fluoride did not directly bind heme iron, unlike catalase in the presence of fluoride alone. It is concluded that the major contribution to the observed ¹H NMR rates comes from rapidly exchangeable protons outside the axial coordination positions of heme iron and that fluoride increases the number and/or decreases the interspin distances of such protons, probably via conformational and/or hydration changes in the catalase protein rather than directly by binding catalase heme.