Hypochlorous acid (HOCl) has both proinflammatory and anti-inflammatory proper- ties, and seems to play an important role in the immune system. The regulation of normal flora contributes to periodontal health, and HOCl seems to have the ability to attack Gram-negative pathogens during periodontitis. Furthermore, high concen- trations of HOCl promote healing by regulating cytokines and growth factors, killing pathogens through chlorination or oxidation, and modulating inflammation through the effects on nuclear factor κB and activator protein-1 of monocytes. After chlo- rination of taurine by HOCl, taurine chloramine is mostly an anti-inflammatory agent and enhances healing. Neither HOCl nor taurine chloramine are common in clinical applications owing to a lack of studies in animal and human models. Both compounds may be suitable as periodontal medication, as they are good antimicrobial agents, inflammation modulators, and healing promoters.

Introduction

Periodontitis is an inflammatory process initiated by plaque biofilm that leads to loss of periodontal attachment to the root surface and adjacent alveo- lar bone, and which ultimately results in tooth loss.1 Destruction of periodontal tissue in periodontal dis- ease is caused by an inappropriate and exaggerated host response to certain microorganisms and their products.2,3 Thus, regulation of the host response, including immune cells and cytokines, is crucial in treating periodontitis.

Free radicals are any species capable of indepen- dent existence that contain one or more unpaired

electrons3 and are able to oxidize a variety of bio- molecules and cell components that are vital to cell and tissue function. Reactive oxygen species (ROS) include not only free radicals but also other reactive species, which are not true radicals but are capable of radical formation in either an intra- or extracellu- lar environment.1 Hypochlorous acid (HOCl), as one of the ROS, is released by neutrophils in periodon- tal pockets, and seems to have a role in regulating inflammation and healing in destroyed gingival and periodontal tissues.2 Yet, therapeutic use of HOCl in periodontitis has not been well studied. Further studies on HOCl as an antimicrobial and health- promoting medication may contribute to a better

• Corresponding author. School of Dentistry, College of Oral Medicine, Taipei Medical University, and Clinical Periodontics, Department of Dentistry, Taipei Medical University Hospital, 250, Wu-Xing Street, Taipei 11042, Taiwan.

Defensive

Destructive ROS

to damage by ROS6 because of their proximity to the ROS generated, the lack of histone proteins to scavenge radicals, and perhaps inefficiencies in the poly(adenosine diphosphate-ribose) polymerase DNA repair mechanism which repairs strand breaks.7

The superoxide radical anion (O •−), the primary product of oxygen metabolism in the mitochon-

Fig. 1 In normal physiologic conditions, the antioxidant (AO) defense system is always superior to the reactive oxygen species (ROS) system.

prognosis for periodontitis treatment. Taurine chlo- ramine (TauCl), a scavenging product of HOCl, has some of the properties of HOCl, and it seems to promote healing.2

Oxidative stress and the redox state

In normal physiology, the dynamic equilibrium be- tween ROS activity and the antioxidant defense sys- tem, which protects and repairs vital tissues, cells and molecular components, shifts to a defensive trend (Fig. 1).

Oxidant stress was defined by Sies4 as “a distur- bance in the pro-oxidant-antioxidant balance in favor of the former, leading to potential damage”. It occurs when there are more ROS than defensive antioxidation, owing to either a reduction in anti- oxidants or an increase in ROS production.1 Direct tissue damage results.

A low redox potential or a reducing environment within periodontal cells and tissues is protective against oxidative stress and helps maintain good health. However, when a periodontal pathogen is present, components or toxins of the pathogen stim- ulate the production of ROS by polymorphonuclear neutrophils (PMNs),5 thus promoting oxidative stress in the periodontal sulcus or pocket.

Origins of ROS in a normal physiologic state

One of the origins of ROS is mitochondria. During cell metabolism, oxygen is consumed in glycolysis to generate energy, forming pyruvate within the mitochondria. Electrons produced from mitochon- drial electron transport systems (respiratory chains) form superoxides at a constant rate as a byproduct of the metabolic pathway. The mitochondrial en- zyme, manganese-dependent superoxide dismutase (MnSOD), scavenges superoxide radicals, converting them into hydrogen peroxide (H2O2). However, mi- tochondrial antioxidant scavengers are sometimes unable to safely take care of all of the ROS produced. Mitochondria are more susceptible than nuclear DNA

drial respiratory chain, is formed by the hexose- monophosphate shunt, via the transfer of one electron to molecular oxygen. Another possible reason for the weak scavenging ability of mitochondria is that they lack catalase. Neutralization of H2O2 into H2O is car- ried out by the enzyme glutathione peroxidase, with the requirement of a coenzyme-reduced glutathione.

Formation of ROS by PMNs

PMNs are the primary mediators of a host’s response to pathogenic microbes during inflammatory diseases. Some of the antimicrobial factors produced by PMNs are capable of preventing bacterial growth through their antibacterial properties.8 During inflammation especially chronic inflammation, inflammatory cells such as activated macrophages and PMNs release various ROS (H2O2, nitric oxide [NO], O •−, and OH•) and HOCl.9 Some of the antimicrobial agents pro- duced by PMNs are ROS, which provide a protective role for the host against inflammation.

Different kinds of mitogens, antigens (e.g., small peptides from bacteria), cytokines and mediators, like granulocyte-macrophage colony-stimulating factor, promote the formation of ROS by PMNs. The activation of the Fcγ receptor by opsonized particles, or toll-like receptors (e.g., TLR-4 and TLR-9) by bacterial DNA triggers ROS formation.10

In a zone of the PMN plasma membrane contacting the phagocytosed particle, O •− is produced by acti- vation of the hexose monophosphate shunt, in which molecular oxygen is catalyzed by NADPH oxidase. This process comprises the respiratory burst within PMNs. The elevated Ca2+ in PMNs, resulting from cell surface receptor stimulation, opens Ca2+-dependent K+channels in the phagolysosome (vacuole) mem- brane. Superoxide is pumped into the vacuole by NADPH oxidase, depolarizing the membrane and generating an ionic gradient. The occurrence of K+influx, creating a hypertonic K+-rich alkaline en- vironment within the vacuole, activates lysosomal proteases, leading to microbial destruction.11

Superoxide radical anions are reduced to H2O2 by superoxide dismutase (SOD; 1 of 3 forms). SOD is localized within human periodontal ligaments and may play a role in defense against excessive superoxide release.12

Hydrogen peroxide, NO, superoxide radicals, and hydroxyl radicals are produced by mitochondria,

leading to elevated numbers and activity of PMNs. In addition to those already mentioned, HOCl can be produced by PMNs during inflammatory conditions.

Effects of ROS on periodontal tissues and cell components

Stimulated by bacterial antigens in periodontal tissue, ROS produced by PMNs increase oxidative damage in gingival tissues, periodontal ligaments, and alveolar bone.13 ROS damage periodontal tis- sues and cell components by depolymerization of extracellular matrix (ECM) components,14,15 lipid peroxidation,16 oxidation of defensive or protective enzymes (e.g., anti-protease),16 increased apoptosis in the deepest area of the sulcular pocket,17 activa- tion of osteoclasts,18,19 induction of proinflammatory cytokines, and DNA damage.16

ECM components

Excessive ROS produced by PMNs destroy ECM com- ponents and alter the metabolic reaction responsi- ble for synthesis of ECM components. Proteoglycans (PGs) and glycosaminoglycans (GAGs), which are some of the ECM components, have been shown to regulate mineralization20,21 and cellular function by their ability to bind growth factors, such as transforming growth factor β.22 Thus, the overpro- duction of ROS degrades PGs by modifying amino acid functional groups, leading to fragmentation of the core protein, while the constituent GAG un- dergoes limited depolymerization. Sulfated GAGs are more resistant than non-sulfated GAGs, such as hyaluronan, to ROS degradation in vitro.23,24

Another important ECM component is type I collagen. Containing 1000 amino acids that form a triple helix, it resists nonspecific proteolytic attacks. ROS directly attack collagen, which results in frag- mentation, and makes it more susceptible to break- down by collagenase.24,25 Superoxide anions and hydroxyl radicals are able to cleave collagen into small peptide fragments at the proline and hydroxy- proline residues.26 Furthermore, lipid peroxidation products (e.g., malondialdehyde) of ROS may inter- act with collagen and alter fibroblast functions.27 This phenomenon can be seen within gingival tis- sues of patients with periodontal disease as lipid peroxidation increases.

Activation of osteoclasts

Although the effects of ROS on bone resorption are not well documented or studied, it has been shown that certain ROS (i.e., superoxide and H2O2)

promote osteoclast formation.28 More substantial evidence supporting this theory is that osteoclasts produce ROS at the ruffle border/bone interface.29,30 Chondroitin sulfate PGs from alveolar bone are par- ticularly susceptible to damage by hydroxyl radicals in vitro.15 In short, ROS may worsen bone destruc- tion in periodontal disease.

DNA damage

Mitochondrial DNA is the most susceptible mole- cule attacked by ROS. Deletions of mitochondrial DNA by ROS are associated with aging and several chronic diseases,31 including chronic periodontitis. Five kilobase of mitochondrial DNA deletions was found in gingival tissues of patients with chronic adult periodontitis.32 Once the mitochondrial DNA is damaged, oxidative stress within a cell can be am- plified owing to an abatement of the expression of proteins which are crucial for electron transport, resulting in cell death.

Transformation and chlorination of H2O2 to HOCl (Fig. 2)

When a pathogen invades periodontal tissues, PMNs are activated. Phagocytosis of the pathogen by PMNs results in assembly and activation of the respiratory burst NADPH oxidase in the phagolysosome mem- brane. The oxidase reduces O2 to O •− and releases O •− into phagolysosomes. The spontaneous dismu- tation or SOD catalysis of two O •− radicals produces O2 and H2O2. Myeloperoxidase catalyzes the oxida- tion of chloride ions by H2O2 to produce HOCl, and the reaction of HOCl with amines and ammonium ions produces toxic chloramines such as monochlo- ramine (NH2Cl). The O2 metabolites (i.e., O •− and H2O2) and products of chloride oxidation (i.e., HOCl and NH2Cl) contribute to the antimicrobial activity of phagolysosomes.

HOCl, the end-product of PMN respiratory bursts, results from the intracellular myeloperoxidase- catalyzed reduction of H2O2 by chlorine during in- flammation.2 Myeloperoxidase is a lysosomal protein stored in the azurophilic granules of neutrophils. Once HOCl is formed by PMNs, it is released extra- cellularly.33 In addition to intracellular myeloper- oxidase, PMNs can secrete HOCl extracellularly.33

Immunologic effects of HOCl

Antibacterial properties of HOCl

HOCl-induced oxidation and/or chlorination may neutralize harmful bacterial endotoxins or exotoxins,

Leukocyte cell membrane

Leukocyte

Superoxide dismutase

2

H2O2

Cl-

Phagolysosome

Leukocyte

cell membrane

Myeloperoxidase

Pathogen

HOCl

• Induces proinflammatory and anti- inflammatory cytokines
• Regulates NF-kB
• Regulates AP-1
• Increase of vascular permeability
• Anti-bacterial property
• Promotes healing and repair
• Regulates TGF-b
• aMacroglobulin oxidation

2

• Neutralization of proinflammatory cytokines and chemokines

Taurine

TauCl

• Induces proinflammatory and anti-inflammatory cytokines
• Regulates NF-kB
• Regulates AP-1
• Anti-bacterial property
• Promotes healing and repair
• Neutralization of proinflammatory cytokines and chemokines

Fig. 2 Transformation and chlorination of hydrogen peroxide (H2O2) to hypochlorous acid (HOCl), and the reaction of HOCl with taurine. The gray boxes summarize the function of HOCl and taurine chloramine (TauCl).

such as lipopolysaccharide (LPS) and gingipains.34 HOCl oxidizes the crucial cysteine residue of the ac- tive site of the gingipains, Rgp and Kgp. Both of them are cysteine proteases of Porphyromonas gingivalis, which lead to the destruction of periodontal tissues; thus, HOCl reduces their potentially harmful activity against periodontal tissues.35

Within physiologic concentration ranges, HOCl has immediate and highly effective microbicidal activity in vivo.36 Various bacterial respiratory electron trans- porters are irreversibly oxidized by HOCl. HOCl may repulse some motile bacteria, especially those with flagella and gliding properties; however, the mech- anism of this repulsive activity remains unclear.37 HOCl is able to enhance the immunogenicity of anti- gens by HOCl chlorination of the proteinaceous parts of the antigens; this promotes the presentation of these proteins to antigen-presenting cells, such as monocytes, macrophages, and dendritic cells.34

Although the mechanism has not yet been clearly elucidated, chlorination of antigens selectively pro- motes a nonspecific immune response against Gram- negative periodontal pathogens, and reduces the response induced by Gram-positive bacteria. This affects the antigen phagocytosis-activated produc- tion of inflammation mediated by macrophages. The chlorination of Gram-positive bacteria-released antigens significantly affects macrophage secretory activities, such as decreasing NO and tumor necrosis factor (TNF)-α; however, phagocytosis and inter- leukin (IL)-6 production remain unchanged.38 This

may be critical in maintaining a normal physiologic state of periodontal tissues. The normal subgingi- val flora in periodontally healthy individuals mainly consists of Gram-positive bacteria, while potent per- iodontal pathogens are mostly Gram-negative bac- teria, such as Porphyromonas gingivalis, Prevotella intermedia, and Actinobacillus actinomycetemcom- itans. As the defensive ability against Gram-negative bacteria is greater than that against Gram-positive bacteria, the immune system seems capable of pro- tecting periodontal tissues by this mechanism. If the amount of TNF-α production is great with normal Gram-positive flora, the destruction may also occur in periodontally healthy individuals, showing the importance of TNF-α in destructive periodontitis.

The antimicrobial activity of HOCl has been ex- tensively studied. It causes respiratory loss in bacte- rial cell membranes due to an irreversible reaction with sulfur- and heme-containing membrane en- zymes and structural proteins,39 leading to cell death and nonviability.40

Dose-dependent inflammatory response modulation

HOCl seems to play a key role in this regulation of proteinase activity, dysregulation of which may lead to periodontitis through a pathway distinct from that of tissue inhibitors of matrix metalloproteinases, and appears to reduce the activity of proteolytic enzymes in a concentration-dependent manner.41,42

Low concentrations of HOCl may be destructive to periodontal tissue. A low dose of HOCl can activate the proforms of matrix metalloproteinases (MMPs), collagenase-2, and gelatinase B via thiol group oxi- dation of their cysteine moiety, while higher con- centrations of HOCl inhibit MMP-7 activation through an oxidative modification of adjacent tryptophan and glycine residues in the catalytic domain.41 Similarly, HOCl inhibits collagenase activities, when the HOCl/ collagenase ratio exceeds 40 (which indicates a high concentration of HOCl). HOCl may also inactivate gelatinases when the HOCl/gelatinase ratio exceeds 30, while it does not seem to inhibit them when the ratio is < 30.43,44

Thus, HOCl possesses both pro- and anti- inflammatory properties depending on the dose. This may be a very important mechanism modulat- ing the inflammatory response within periodontal tissues. HOCl is capable of mediating the generation of histamine N-chloramines, and thus may modulate histamine activity, tissue distribution, and metab- olism within sites of inflammation.45 Chemotactic mediators enhance leukocyte adherence to activated endothelium and in situ diapedesis.

Furthermore, HOCl neutralizes various proinflam- matory cytokines and chemokines (chemotactic factors, leukotrienes, TNF-α, IL-1β, IL-2, and IL-6), regulates metalloproteinases, and releases activated growth factors. These reactions may be due to either a direct oxidation of crucial thiol or thioether residue(s) of the molecules or an indirect modula- tory effect on the capacity of α2-macroglobulins to bind them.

α2-Macroglobulins are plasma molecules that bind to and neutralize proteases, cytokines, and growth factors. In plasma in a normal physiologic state, α2-macroglobulin’s binding affinity is greater to- ward growth factors (e.g., TGF-β, basic fibroblast growth factor [also called FGF-2], β-nerve growth factor [β-NGF], and platelet-derived growth factor [PDGF], with dissociation constant (Kd) values in a na- nomolar range) than to cytokines (e.g., TNF-α, IL-1β, IL-2, IL-6, IL-8, with Kd values in a micromolar range), which leads to the activities of cytokines being more predominant than those of growth factors. Conse- quently, 85−90% of TGF-β and PDGF molecules are inactivated by being bound to α2-macroglobulins. When α2-macroglobulin is oxidized by HOCl, it tends to undergo repair and fibrosis, owing to a decrease in protease binding, an important increase in α2- macroglobulin affinity for destructive or inflam- mation-induced factors (TNF-α, IL-2, and IL-6), and a greater decrease in affinity to growth fac- tors (β-NGF, PDGF-BB, TGF-β1, and TGF-β2). In ad- dition, after being oxidized by HOCl, the oxidized α2-macroglobulin−methylamine complex leads to a decrease in the binding of various growth factors,

resulting in increases in free growth factors, with no modification of its affinity to inflammatory cytokines.46

However, HOCl may exert a deleterious stimula- tion of inflammatory processes. As mentioned above, low concentrations of HOCl may activate the pro- forms of MMPs, gelatinase B, and collagenases. HOCl may also inhibit α2-macroglobulin-related neutral- ization of cell proteases, whereas HOCl inactivates the α1-proteinase inhibitor.46,47 HOCl may also in- terfere with the C5 component of the complement cascade which, upon activation, generates two fragments: the C5b fragment with antibacterial membrane-lytic activity, and the C5a fragment with PMN chemotactic properties. HOCl-induced oxi- dation of methionine residues in the C5 fragment generates structural changes that result in its acti- vation.48 HOCl promotes macrophage adherence to the endothelium and enhances endothelial perme- ability.49 HOCl promotes an innate immune response against Gram-negative bacteria (unlike Gram- positive species) via chlorination of antigens.38

In conclusion, unlike natural TNF-α or TNF-β, HOCl has a double-edged effect in periodontitis. Although the effects of HOCl in periodontitis are not well documented, HOCl seems to play a very important role in modulating the progress of peri- odontitis and may help in repairing its damage. With further studies and evidence, HOCl may become a drug used in surgery or postsurgical medication.

Enhanced cell proliferation and extracellular component production by HOCl

The interruption of tissue destruction, which pro- motes healing, is brought about by direct neutrali- zation and cellular inhibition of proinflammatory mediators. HOCl also induces the production of cellular growth factors, such as insulin-like growth factor, epidermal growth factor, keratinocyte growth factor (also called FGF-7), FGF-1, FGF-2, TGF-β, PDGF, vascular endothelium growth factor, connec- tive tissue growth factor, and/or cementum-derived growth factor.50 These molecules also promote peri- odontal tissue regeneration. Signal transduction enzymes are involved in the synthesis of some of the above molecules.

Furthermore, HOCl can activate the transforming growth factor TGF-β, a reparative mediator which promotes tissue repair and fibrosis. Native TGF-β consists of two peptides: an N-terminal latency- associated peptide and a C-terminal mature TGF-β. HOCl-induced latency-associated peptide oxidation may facilitate access to the active site of the ma- ture TGF-β molecule, resulting in its activation.

HOCl may also affect the signal transduction pathway in inflammatory cytokine formation. The nuclear factor κB and activator protein (AP)-1 are redox-sensitive transcription factors, the control of which has been proposed as a potentially important host-modulation strategy in periodontitis.51 NF-κB and AP-1 are stimulated by a specific mitogen- activated protein (MAP) kinase pathway, which con- sists of a cascade of transduction signals activated by a membrane receptor-linked protein tyrosine kinase. In short, their activation and phosphoryla- tion in periodontal tissues cause periodontitis.

NF-κB is an important signal transduction pro- tein responsible for the genetic transcription of many inflammatory mediators, including those regulat- ing inflammation, acquired immunity, cell-to-cell interactions, cell apoptosis, and proliferation (e.g., IL-1α, IL-1β, IL-2, IL-6, TNF-α, NO, prostaglandin E2, TGF-β, and adhesion molecules), as well as in- hibitors of apoptosis proteins. Inactive NF-κB forms a dimer with an IκB inhibitory protein in the cyto- plasm, which masks its nuclear location signal site and latently stabilizes NF-κB. NF-κB is activated by specific MAP kinases, IκB kinases (IKKs). This in- ducible serine phosphorylation leads to a polyubi- quitination of adjacent lysines in the IκB inhibitory protein, which is a signal of degradation in the 26S proteasome pathway. This releases and activates NF-κB, which is translocated to the nucleus and binds to DNA for the transcription of inflammatory proteins. Activation of NF-κB by receptor-activated nuclear factor-κB ligand in monocytes may promote bone resorption through cytokine production. In short, NF-κB is a major molecule promoting inflam- mation in periodontitis.

AP-1 is a two-gene-dependent transcription fac- tor (Jun and Fos), and is capable of producing some cytokines (e.g., IL-8) and MMPs involved in perio- dontitis. The monomers (c-Jun, c-Fos, v-Jun, v-Fos, Fos, Fra-1, Fra-2, Jun-D, Jun-B, and ATF) can gen- erate a homodimeric complex (Jun/Jun) or a het- erodimeric complex (Jun/Fos). This transcription factor family is critical to the early genetic regula- tion of immune responses. AP-1 is activated by an- other specific MAP kinase, c-Jun N-terminal kinase (JNK). Similar to IKK, JNK phosphorylates c-Jun, fol- lowing TNF-α-receptor stimulation, thereby inducing AP-1 activation.

All of the mediators and proteins mentioned above are involved in periodontal diseases, and their physiologic inhibition seems to be crucial to perio- dontal tissue turnover, and to triggering the pro- cesses of regeneration.52 In contrast to H2O2, HOCl does not significantly activate JNK, except at a le- thal dose (50 μM), thus preventing the tissue destruc- tion caused by MMPs and IL-8. The extremely low doses (20−50 μM) of HOCl required for MAP kinase

activation, compared with H2O2 (400−1000 μM), may be due to the high reactivity of HOCl with thiol groups and not to physiologic enzymatic degradation.53

A toxic concentration of HOCl oxidizes and causes irreversible loss of intracellular protein thiol groups, including glutathione, glutaredoxin and thioredoxin, resulting in their cross-linking and aggregation.54 Low doses of HOCl oxidize preferentially accessi- ble thiol residues of cysteine amino acids of vital cellular antioxidants, such as reduced glutathione, thioredoxin and glutaredoxin.

Thioredoxin expression modulates NF-κB activity at three levels. In the nucleus, it helps NF-κB bind to DNA.55 In the cytosol, it activates NF-κB at a down- stream level of NF-κB-inducing kinase;56 while near cell membranes, it inhibits NF-κB-mediated cytokine production at a level upstream of NF-κB-inducing kinase and at a level downstream of TNF receptor- associated factor protein.57 HOCl may oxidize cyto- plasmic antioxidants close to the cell membrane, and thioredoxin may thus be unable to inhibit NF-κB- mediated cytokine production in this case. There- fore, HOCl here increases inflammatory mediator release.

In contrast to thioredoxin, glutaredoxin expres- sion increases NF-κB activation and, like thioredoxin, increases AP-1 activation. HOCl oxidation in these cases seems to be anti-inflammatory.

Nonspecific oxidants, like H2O2 and HOCl, may modulate IκB kinase activities, which then induces phosphorylation of the tyrosine residue 42 instead of the serine residues 32/36 of IκB-α, triggering the dissociation of IκB-α from the NF-κB dimer. Interest- ingly, IκB-α dissociates without degradation by the 26S proteasome mentioned above.58 In conclusion, HOCl may indirectly regulate these transcription factors and inflammation.

Thus, although HOCl has the ability to inhibit redox-sensitive transcription factors, similarly to TauCl, antioxidant-mediated HOCl neutralization prevents this activity in vivo. In fact, moderate HOCl- induced depletion of antioxidants may favor HOCl- mediated nonspecific activation of protein tyrosine kinases, i.e., MAP kinases, which leads to nonspe- cific proinflammatory gene transcription. Therefore, with the mechanism affecting α2-macroglobulins, nontoxic HOCl concentrations induce cell prolifer- ation and stimulate ECM component production in human fibroblasts.59

Influence of TauCl, which scavenges production of HOCl, in periodontal disease

TauCl, a product of the neutrophil myeloperoxidase- halide system, formed by a reaction of taurine with

HOCl, is known to be a long-lived antimicrobial and anti-inflammatory oxidant.60 As it can be formed by HOCl, it reveals some of the same properties. Nevertheless, it has more potent anti-inflammatory effects.

TauCl exerts a direct concentration-dependent inactivation of type VII collagenases,43,44 inacti- vates α1-proteinase inhibitors46,47 like HOCl, re- duces the HOCl-mediated increase in vascular permeability,49 significantly inhibits the in vitro cell production of various inflammatory mediators and ROS (e.g., IL-1β, IL-6, and IL-8) in LPS-stimulated human adherent monocytes, inhibits lymphocyte proliferation,61 interferes with transduction signals which generate MMP-9 expression in LPS-stimulated murine peritoneal macrophages,62 inactivates NF-κB by reducing the translocation of NF-κB and its DNA- binding activities, inhibits the production of MCP-1, MIP-2, IL-1β, IL-2, IL-6, IL-8, TNF-α, NO, and pros-

taglandin E2 due to the involvement of the NF-κB and AP-1 transcriptional pathways,34,63 inhibits the NF-κB-related transcription of inducible nitric oxide synthase and TNF-α genes in a rat model of broncho- alveolar macrophages,64 and inducible nitric oxide synthase, cyclooxygenase-2, TNF-α, MCP-1 and MIP-2 genes in rat cortical astrocytes,63 and inhibits cy- clooxygenase-2 gene NF-κB post-transcriptional events which are more accessible than transcription, implying that the inhibition is mainly at the post- transcriptional level.65 Similarly, TauCl essentially suppresses the translation of TNF-α mRNA61 and induces inhibition of IKK activity by maintaining un- phosphorylated cytoplasmic IκB-α. TauCl reduces IKK activation at a downstream rather than an up- stream level in the kinase cascade. Instead of ser- ine 32/36 phosphorylation, TauCl induces oxidation of IκB-α methionine 45, yielding a sulfoxide residue. This oxidation is likely to induce a spatial structural change that masks serine 32/36, preventing phos- phorylation, or avoids phosphorylated IκB-α recog- nition by F-box proteins and subsequent lysine 21/22 ubiquitination.66 Consequently, it inhibits the acti- vation of NF-κB and degradation of IκB-α. The above information is summarized in Table 1.

Therefore, TauCl has the ability to inhibit the production of the principal inflammatory mediators involved in the pathogenesis of periodontitis. These inhibitions may involve activities at the level of gene transcription, at the post-transcriptional stage, and/ or at mRNA translation. Consequently, TauCl not only protects tissues against excess HOCl (an antioxidant effect), but also possesses better anti-inflammatory and healing promotion properties.

The successful wound healing performance of HOCl was proven by Selkon et al.,67 who evaluated the effect of HOCl in treating chronic venous leg ulcers. They determined that using HOCl washes as

an adjunctive therapy for recalcitrant venous leg ulcers appreciably increased healing and rapidly relieved pain. HOCl washes were given over 12 weeks to patients who failed to achieve a 44% ulcer reduc- tion after 3 weeks of standard treatment. Encouraging results were shown after 3 weeks in which 45% of 20 ulcers had healed and a further 25% were reduced by over 60% in size.

However, adversely, HOCl did not promote wound healing compared with electrolyzed water in another study.68 The author also quoted from the study of Kozol et al. that sodium hypochlorite (NaOCl) had toxic effects on wound healing (e.g., on neutrophils, fibroblasts, and endothelial cells even at dilute con- centrations of 0.0005 − 0.025%).69 Wounds created on mouse ears and treated with 0.25% NaOCl showed delayed epithelialization and neovascularization.70 The materials used in those experiments were not pure acidic HOCl, but alkaline NaOCl or HOCl in a saline solution. Thus, one cannot conclude that HOCl is detrimental to healing based on their conclusions. Stabilized, healing-promoting HOCl should be kept at a pH of 3.5 − 5 in order to maintain its de- sired activity.71 HOCl underwent antibacterial and wound healing tests, and it was found that the min- imal bactericidal concentration of HOCl for most bacteria used in those experiments was < 3 μg/mL at room temperature for 60 minutes, except for Aspergillus niger, which required 86.6 μg/mL. The time required for bacteria to be killed was the least (within 1 minute), and it had the greatest relative therapeutic index when OCl− was compared with H2O2. In the wound healing test, healing was more obvious after the use of HOCl than saline, and atrau- matic wiping with HOCl between dressing applica- tions seemed to favor healing. Although in that research, specific periodontal pathogens were not included; it still indicated the importance of HOCl in wound healing and the antibacterial properties

of HOCl.

Conclusion

Excessive ROS seem to be harmful to human tissues, while HOCl is able to modulate the inflammatory response, often in a concentration-dependent man- ner, and may have both proinflammatory and anti- inflammatory characteristics. The anti-inflammatory effects would appear to predominate, but the out- comes of these multiple effects in vivo require fur- ther exploration. TauCl, however, possesses mostly anti-inflammatory properties, and may promote healing and alleviate inflammation.

Based on the above viewpoints, the Periodontal Department of Taipei Medical University Hospital began using an ultrasonic spray of HOCl (HSP-600SME;

Table 1. Summary of extracellular effects of hypochlorous acid (HOCl) and taurine chloramine (TauCl)

HOCl                                                       TauCl*

Effects                            Inflammation-     Healing-     Effects                        Healing- enhancing      promoting                       promoting

Type VII collagenases    High concentration:                   ↓                            ↑               Direct concentration:         ↑

inhibition                                                                dependent

Low concentration:                                                    inactivation

promotion

cytokines and chemokines

NF-κB activation          Present (due to oxidation             ↑                            ↓               Inhibit                             ↓

of thioredoxin and phosphorylation of IκB-α)

Inhibit (due to oxidation of glutaredoxin)

IκB kinase                  Not stated                                                              Inhibit                             ↓

activation

free growth factor)

Activation of TGF-β         Present                                                       ↑               Not stated

• TauCl seems to promote the healing of periodontitis. ↑ = promotes; ↓ = inhibits.

Biotech Corp., Taiwan) for constant sterilization and infection control of clinical cubicles and wound irrigation during periodontal surgery in 2007. Appli- cation of HOCl through an ultrasonic delivery sys- tem may prove to be a good modality for preventing infections in hospitals.

Acknowledgments

Special thanks to Jenny U. K. I. and Hin F. for proof- reading and corrections.

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