pH-dependent inhibition by azide and fluoride of the iron superoxide dismutase from Propionibacterium shermanii.
The iron-containing superoxide dismutase from Propionibacterium shermanii shows, in contrast with other iron superoxide dismutases, only a minor 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 not diminished under the same conditions. The binding constant between azide 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 iron superoxide dismutases. This seems to be a discrepancy because all other iron superoxide dismutases so far known are described as being inhibited by 50-70% by 10 mM 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 still bound to the Fe3+ at the active site but their inhibition capacity decreased. This observation implies that both anions bind to the metal 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 superoxide anion binds to the same position and has to replace the sixth ligand, leading to a diminished catalytic activity of the superoxide dismutase owing to steric and/or electrostatic inhibition of the ligand.
Meier
B
B
Fakultät für Physik E17, Technische Universität München, James Franck Strasse, D-85747 Garching, Federal Republic of Germany.
Scherk
C
C
Schmidt
M
M
Parak
F
F
D001386:Azides / Q000378:metabolism / Q000494:pharmacology*; D001665:Binding Sites; D002942:Circular Dichroism; D004578:Electron Spin Resonance Spectroscopy; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005290:Ferric Compounds / Q000378:metabolism; D005459:Fluorides / Q000378:metabolism / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism; D046911:Macromolecular Substances; D008345:Manganese / Q000378:metabolism; D011424:Propionibacterium / Q000201:enzymology*; D013482:Superoxide Dismutase / Q000037:antagonists & inhibitors*
D001386:Azides; D004791:Enzyme Inhibitors; D005290:Ferric Compounds; D046911:Macromolecular Substances; D008345:Manganese; D007501:Iron; D013482:Superoxide Dismutase; D005459:Fluorides