FLUORIDE 31 (1)
 1998, p. 43-45		International Society for Fluoride Research Table of Contents
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INTERACTIONS BETWEEN FLUORIDE AND
BIOLOGICAL FREE RADICAL REACTIONS

R Rzeuski, D Chlubek and Z Machoy

The term "free radical" means an atom, molecule or its fragment containing an unpaired electron. Oxygen free radicals include the superoxide radical (·O2–) and hydroxyl radical (OH·) which, together with hydrogen peroxide (H2O2) and singlet oxygen (1O2), are jointly called reactive oxygen species. They originate in cells in the microsomal and mitochondrial electron transport chains, in chloroplasts in the process of photosynthesis, and in numerous enzymatic reactions as well as autoxidation of many compounds. 1 Due to their high reactivity they may lead to chemical modification and impairment of the components of living cells (proteins, lipids, carbohydrates, nucleotides). 2

Aerobic organisms mobilize mechanisms protecting them against the toxic action of oxygen radicals. Among these the most important involve enzymes, including superoxide dismutase (SOD), which catalyzes superoxide radical dismutation: ·O2– + ·O2– + 2H+ right arrow H2O2 + O2. The resulting hydrogen peroxide in turn is decomposed by the enzyme glutathione peroxidase (GSH-Px) and catalase (CAT).

The basic activity of neutrophilic granulocytes - polymorphonuclear (PMN) leukocytes - is to defend the organism against infections. Fluoride is known to stimulate the so-called respiratory burst and the production of superoxide radicals of human and rabbit neutrophils as well as those of guinea pig.3-5 This process is associated with the activation of NADPH-dependent membranous oxidase appearing in these cells. Analogous effects are evoked by fluoride ions in leukemic cells (HL-60).6

Similar effects are exerted by fluoride stimulation in human PMN leukocytes, whereas OH· is also produced alongside ·O2–, while added exogenous SOD and/or CAT do not overcome these effects. Moreover, high fluoride concentrations are likely to inhibit SOD. Production of OH· and ·O2– radicals is dependent on fluoride concentration; superoxide radicals prevail at high fluoride concentrations, whereas at low ones there is dominance of hydroxyl radicals generated in the Haber-Weiss reaction with the participation of Fe2+ as well as H2O2 and ·O2–7,8. Decrease in the activity of free radical-scavenging enzymes (SOD, GSH-Px) also occurs in people living in areas of endemic fluorosis.9 A similar inhibitory effect of fluoride on seed germination has been observed by Wilde and Yu. They are of the opinion that the toxicity of free radicals and H2O2 is greater if fluoride can impair the production of free radical scavengers such as ascorbic acid and glutathione, or the functioning of protective enzymes such as ascorbate peroxidase and SOD.10

Isolated rat mast cells incubated with fluoride release histamine and produce oxygen radicals.11 Stimulation of superoxide radical production by fluoride occurs to such an extent that it is not inhibited even by added adenosine, abolishing the influence on neutrophils by the well-known inductor of peroxide production, which is N-formylmethionylleucylphenylalanine (fMLP).12 In a study by Bell et al the anti-inflammatory drug diclofenac inhibited superoxide generation induced by serum-treated zymosan (STZ) and fluoride anion, but not by phorbol myristate acetate (PMA) in vitro. Following treatment of patients with rheumatoid arthritis, inhibition of superoxide production occurred when STZ and PMA, but not fluoride, were used as stimuli.13

In rat liver macrophages fluoride ions elicit a release of arachidonic acid and prostaglandins but not formation of inositol phosphates or superoxide. The effects of fluoride require extracellular calcium and are inhibited by staurosporine and by phorbol ester treatment of the cells.14 Similar results have been observed with respect to leukotrienes in human neutrophils.15 The cascade of arachidonic acid begins by lipoxygenation, while when the resulting endoperoxides of fatty acids in the reaction with H2O2 produce hydroxyl radicals.16 Oxygen radicals can damage living cells by intensifying lipid peroxidation, leading to the formation of malondialdehyde which is inhibited by fluoride.17

Escherichia coli mutants deprived of superoxide dismutase are unable to grow in aerobic environment, because there is a fall in the activity of 6-phosphogluconate dehydratase. Incubation of these mutants with fluoride protects this enzyme against the action of superoxide radicals. Practically 100% protection was achieved at a fluoride concentration of 10 mM; at a concentration of 0.2 mM fluoride protection was close to 100% at the beginning and around 75% after one hour. Fluoride ions also protect this enzyme against the action of such chemical species as ferricyanide, diamide, and nitrite, as well as hydrogen peroxide.18

The process of photosynthesis together with the accumulation of energy in the form of ATP (adenosine triphosphate) proceeds with the participation of systems transporting electrons that produce radicals formed during monoelectron passages. The simultaneous production of oxygen occurs by photolysis of water. Substitution of chloride ions, appearing in the photosystem II, by fluoride ions inhibits the photooxidation of water and increases production of new radical forms in protein chains of this system, unable to commence the process of photolysis.19

Farhangrazi et al are of the opinion that oxidation of horseradish peroxidase by IrCl62– is significantly accelerated in the presence of fluoride. This acceleration results in the formation of a new compound which is a ferric-fluoride complex containing a porphyrin pi-cation radical.20

Radicals may also form with the participation of ionizing radiation. Irradiating crystals of fluoroapatite containing various amounts of fluoride produces different radical signals in ESR spectroscopy. Doublets and singlets were observable up to 1.82% fluoride content. At higher fluoride levels, both doublets and singlets were barely discernible until, at 3.69% fluoride (essentially pure fluoroapatite), a weak but clear singlet was again observed.21

Despite numerous reports, the relationship between fluoride in free radical reactions remains unclear and requires further investigations.

REFERENCES

  1. Cross CE, Halliwell B, Borish ET et al. Oxygen radicals and human disease. Annals of Internal Medicine 107 526-545 1987.
  2. Ciechanowski K. On free radicals in medicine. Polski Tygodnik Lekarski 42 939-941 1987.
  3. Della Bianca V, Grzeskowiak M, Dusi S, Rosi F. Fluoride can activate respiratory burst independently of Ca2+ stimulation of phosphoinositide turnover, and protein kinase C translocation in primed human neutrophils. Biochemical and Biophysical Research Communications 150 955-964 1988.
  4. Bober J. Effect of fluoride ion on oxygen metabolism and function of human and rabbit neutrophil granulocytes. PhD thesis, Pomeranian Medical Academy, 1992.
  5. Toper R, Aviram A, Aviram I. Fluoride mediated activation of guinea pig neutrophils. Biochimica et Biophysica Acta 931 262-266 1987.
  6. Klein JB, Scherzer JA, McLeish KR Interferon-gamma enhances superoxide productions by HL-60 cells stimulated with multiple agonists. Journal of Interferon Research 11 69-74 1991.
  7. Wang YY, Zhao BL, Li XJ et al. Spin trapping technique studies on active oxygen radicals from human polymorphonuclear leukocytes during fluoride-stimulated respiratory burst. Fluoride 30 (1) 5-15 1997.
  8. Zhao BL, Li XJ, Xin WJ. ESR studies on active oxygen radicals produced in the respiratory burst of human polymorphonuclear leukocytes. Cell Biology International Reports 13 529-536 1989.
  9. Li J, Cao S. Recent studies on endemic fluorosis in China. Fluoride 27 (3) 125-128 1994.
  10. Wilde LG, Yu MH. Effect of fluoride on superoxide dismutase (SOD) activity in germinating mung bean seedlings. Fluoride 31 (2) 1998 (in press).
  11. Hong XJ, Francker A, Diamant B. Effects of N-acetylocysteine on histamine release by sodium fluoride and compound 48/80 from isolated rat mast cells. International Archives of Allergy and Applied Immunology 96 338-343 1991.
  12. Burkey TH, Webster RO. Adenosine inhibits fMLP-stimulated adherence and superoxide anion generation by human neutrophils at an early step in signal transduction. Biochimica et Biophysica Acta 1175 312-318 1993.
  13. Bell AL, Adamson H, Kirk F et al. Diclofenac inhibits monocyte superoxide production ex vivo in rheumatoid arthritis. Rheumatology International 11 27-30 1991.
  14. Schulze-Specking A, Duyster J, Gebicke-Haerter PJ et al. Effect of fluoride, pertussis and cholera toxin on the release of arachidonic acid and the formation of prostaglandin E2, D2, superoxide and inositol phosphates in rat liver macrophages. Cellular Signalling 3 599-606 1991.
  15. Brom C, Köller M, Brom J, Konig W. Effect of sodium fluoride on the generation of lipoxygenase products from human polymorphonuclear granulocytes, granulocytes, mononuclear cells and platelets - indication for the involvement of G proteins. Immunology 68 240-246 1989.
  16. Nave JF, Jacobi D, Gaget C et al. Evaluation of 5- and 6-fluoro derivatives of arachidonic acid and 5,8,14-eicosatrienoic acid as substrates and inhibitors of 5 lipoxygenase. Biochemical Journal 278 549-555 1991.
  17. Shayiq RM, Raza H, Kidwai AM. Fluoride and lipid peroxidation: a comparative study in different rat tissues. Bulletin of Environmental Contamination and Toxicology 37 70-76 1986.
  18. Gardner PR, Fridovich I. Superoxide sensitivity of the Escherichia coli 6-phosphogluconate dehydratase. Journal of Biological Chemistry 266 1478-1483 1991.
  19. Baumgarten M, Philo JS, Dismukes GC. Mechanism of photoinhibition of photosynthetic water oxidation by Cl– depletion and F– substitution: oxidation of a protein residue. Biochemistry 29 10814-10822 1990.
  20. Farhangrazi ZS, Sinclair R, Powers L, Yamazaki I. Formation of a porphyrin pi-cation radical in the fluoride complex of horseradish peroxidase. Biochemistry 34 14970-14974 1995.
  21. Peckauskas RA, Pullman I. Radiogenic free radicals in apatite: the influence of fluoride and hydroxide. Calcified Tissue Research 21 21-129 1976.


Authors: Department of Biochemistry and Chemistry, Pomeranian Medical Academy,
Al. Powstanców Wielkopolskich 72, 70-111 Szczecin, Poland

FLUORIDE 31 (1)
 1998, p. 43-45		 International Society for Fluoride Research
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