pH-dependent inhibition by azide and fluoride of the iron superoxide dismutase from Propionibacterium shermanii

Abstract

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.

Stopped-flow Fluorescence Studies of Inhibitor Binding to Tyrosinase from Streptomyces antibioticus

“…superoxide dismutase (50), urease (51), laccase (52), peroxidase (53), and enolase (54)), often with apparent inhibition constants in the micromolar to millimolar range. In most cases, the fluoride binding is pH-dependent, and it is thought that fluoride replaces a metal-bound hydroxide ion or water ligand.…”

Fluoride Exposure Attenuates Expression of Streptococcus pyogenes Virulence Factors

“…None of these studies examined the effect of fluoride on S. pyogenes and cannot be used to confirm our findings. Many previous studies have determined that fluoride inhibits the function of several proteins, including catalase, superoxide dismutase, and elongation factor G (37,38). These studies did not examine the effect of fluoride on the amount expressed of these proteins.…”

Antioxidants: An Overview on the Natural and Synthetic Types

“…101 All types of SOD bind single charged anions such as azide and fluoride, but distinct differences have been noted in the susceptibilities of Fe, Mn or Cu/Zn-SODs. Cu/Zn-SOD is competitively inhibited by N3 -, CN -, 102 and by F -.…”