PH-DEPENDENT INHIBITION BY AZIDE AND FLUORIDE OF THE IRON SUPEROXIDE-DISMUTASE FROM PROPIONIBACTERIUM-SHERMANII

The iron-containing superoxide dismutase from Propionibacterium sherma nii shows, in contrast with other iron superoxide dismutases, only a m inor inhibition by azide or fluoride (10-100 mM) of up to 23% at pH 7. 8. The activity of the protein with Mn bound to the active site was no t diminished under the same conditions. The binding constant between a zide and the Fe3+ ion was determined as approx. 2 mM and for fluoride approx. 2.3 mM; they are so far comparable to those known for other ir on superoxide dismutases. This seems to be a discrepancy because all o ther iron superoxide dismutases so far known are described as being in hibited by 50-70% by 10mM azide. However, towards lower pH there was a drastically increased inhibition by both anions. At pH 6.8 about 80% inhibition was exhibited by azide or fluoride at a concentration of 10 mM or higher. In contrast, on increasing the pH, azide or fluoride st ill bound to the Fe3+ at the active site but their inhibition capacity decreased. This observation implies that both anions bind to the meta l at a position that is empty at low pH, whereas at higher pH water or a negatively charged hydroxyl anion is bound. It is likely that the s uperoxide anion binds to the same position and has to replace the sixt h ligand, leading to a diminished catalytic activity of the superoxide dismutase owing to steric and/or electrostatic inhibition of the liga nd.