The indoleamine melatonin as a free radical scavenger, electron donor, and antioxidant. In vitro and in vivo studies

Melatonin facts: Lack of evidence that melatonin is a radical scavenger in living systems

Melatonin is a small natural compound, so called a neuro-hormone that is synthesized mainly in pineal gland in animals. Its main role is to master the clock of the body, under the surveillance of light. In other words, it transfers the information concerning night and day to the peripheral organs which, without it, could not "know" which part of the circadian rhythm the body is in. Besides its main circadian and circannual rhythms mastering, melatonin is reported to be a radical scavenger and/or an antioxidant. Because radical scavengers are chemical species able to neutralize highly reactive and toxic species such as reactive oxygen species, one would like to transfer this property to living system, despite impossibilities already largely reported in the literature. In the present commentary, we refresh the memory of the readers with this notion of radical scavenger, and review the possible evidence that melatonin could be an in vivo radical scavenger, while we only marginally discuss here the fact that melatonin is a molecular antioxidant, a feature that merits a review on its own. We conclude four things: (i) the evidence that melatonin is a scavenger in acellular systems is overwhelming and could not be doubted; (ii) the transposition of this property in living (animal) systems is (a) theoretically impossible and (b) not proven in any system reported in the literature where most of the time, the delay of the action of melatonin is over several hours, thus signing a probable induction of cellular enzymatic antioxidant defenses; (iii) this last fact needs a confirmation through the discovery of a nuclear factor-a key relay in induction processes-that binds melatonin and is activated by it and (iv) we also gather the very important description of the radical scavenging capacity of melatonin in acellular systems that is now proven and shared by many other double bond-bearing molecules. We finally discussed briefly on the reason-scientific or else-that led this description, and the consequences of this claim, in research, in physiology, in pathology, but most disturbingly in therapeutics where a vast amount of money, hope, and patient bien-être are at stake.

Melatonin as Modulator for Sulfur and Nitrogen Mustard-Induced Inflammation, Oxidative Stress and DNA Damage: Molecular Therapeutics

Sulfur and nitrogen mustards, bis(2-chloroethyl)sulfide and tertiary bis(2-chloroethyl) amines, respectively, are vesicant warfare agents with alkylating activity. Moreover, oxidative/nitrosative stress, inflammatory response induction, metalloproteinases activation, DNA damage or calcium disruption are some of the toxicological mechanisms of sulfur and nitrogen mustard-induced injury that affects the cell integrity and function. In this review, we not only propose melatonin as a therapeutic option in order to counteract and modulate several pathways involved in physiopathological mechanisms activated after exposure to mustards, but also for the first time, we predict whether metabolites of melatonin, cyclic-3-hydroxymelatonin, N1-acetyl-N2-formyl-5-methoxykynuramine, and N1-acetyl-5-methoxykynuramine could be capable of exerting a scavenger action and neutralize the toxic damage induced by these blister agents. NLRP3 inflammasome is activated in response to a wide variety of infectious stimuli or cellular stressors, however, although the precise mechanisms leading to activation are not known, mustards are postulated as activators. In this regard, melatonin, through its anti-inflammatory action and NLRP3 inflammasome modulation could exert a protective effect in the pathophysiology and management of sulfur and nitrogen mustard-induced injury. The ability of melatonin to attenuate sulfur and nitrogen mustard-induced toxicity and its high safety profile make melatonin a suitable molecule to be a part of medical countermeasures against blister agents poisoning in the near future.

The effect of n-3/n-6 polyunsaturated fatty acids on acute reflux esophagitis in rats

BACKGROUND

Polyunsaturated fatty acids (PUFAs) play various roles in inflammation. However, the effect of PUFAs in the development of reflux esophagitis (RE) is unclear. This study is to investigate the potential effect of n-3/n-6 PUFAs on acute RE in rats along with the underlying protective mechanisms.

METHODS

Forty Sprague Dawley rats were randomly divided into four groups (n = 10 in each group). RE model was established by pyloric clip and section ligation. Fish oil- and soybean oil-based fatty emulsion (n-3 and n-6 groups), or normal saline (control and sham operation groups) was injected intraperitoneally 2 h prior to surgery and 24 h postoperatively (2 mL/kg, respectively). The expressions of interleukin (IL)-1β, IL-8, IL-6 and myeloid differentiation primary response gene 88 (MyD88) in esophageal tissues were evaluated by Western blot and immunohistochemistry after 72 h. The malondialdehyde (MDA) and superoxide dismutase (SOD) expression in the esophageal tissues were determined to assess the oxidative stress.

RESULTS

The mildest macroscopic/microscopic esophagitis was found in the n-3 group (P < 0.05). The expression of IL-1β, IL-8, IL-6 and MyD88 were increased in all RE groups, while the lowest and highest expression were found in n-3 and n-6 group, respectively (P < 0.05). The MDA levels were increased in all groups (P < 0.05), in an ascending trend from n-3, n-6 groups to control group. The lowest and highest SOD levels were found in the control and n-3 group, respectively (P < 0.05).

CONCLUSION

n-3 PUFAs may reduce acute RE in rats, which may be due to inhibition of the MyD88-NF-kB pathway and limit oxidative damage.

Melatonin actions on ovaprim (synthetic GnRH and domperidone)-induced oocyte maturation in carp

The major objective of the present study was to demonstrate the actions of exogenous melatonin on ovaprim (synthetic GnRH and domperidone)-induced final oocyte maturation focusing on the oxidative status of pre-ovulatory follicles in the carpCatla catla. Accordingly, gravid carp during the early spawning phase of the reproductive cycle were injected with melatonin and/or ovaprim at different time intervals or luzindole (a pharmacological blocker of melatonin receptors) before their administration. We studied their effects on the latency period, the rate of germinal vesicle breakdown (GVBD; a visual marker of final oocyte maturation) in oocytes, and the levels of maturation-promoting factor (MPF), as well as oxidative stress, different antioxidants, melatonin and MT1 melatonin receptor protein in the extracts of pre-ovulatory follicles. Notably, melatonin treatment 2 h before the injection of ovaprim resulted in the shortest latency period as well as the highest rate of GVBD and MPF formation. Exogenous melatonin, irrespective of the injection schedule, caused a significant reduction in intra-follicular oxidative stress and an increase in the levels of both enzymatic and non-enzymatic antioxidants, melatonin and its receptor protein. Concentrations of ovarian melatonin in each fish exhibited a significant negative correlation with the level of oxidative stress, but a positive correlation with the rate of GVBD and the activity/level of different antioxidants. However, no significant effects of melatonin and/or ovaprim were detected in luzindole-pretreated carp. Collectively, the present study provides the first evidence that melatonin pretreatment in carp ameliorates ovaprim actions on the process of final oocyte maturation by the formation of MPF and alleviates oxidative stress in pre-ovulatory follicles by stimulating different antioxidants.

Perioperative melatonin use

Melatonin is a substance chiefly produced by the pineal gland and has a key role in the sleep-wake cycle. It also has an important antioxidant role. Exogenous melatonin has a short half-life and is available in a range of preparations. Newer analogues targeted for the recently discovered melatonin MT1 and MT2 receptors have also been developed. Exogenous melatonin is used as a resynchronisation agent in jet lag and for other sleep disturbances. Perioperatively, melatonin has been used as a premedicant, sedative and analgesic. It decreases paediatric emergence delirium. The antioxidant properties of melatonin are being investigated for use in sepsis and reperfusion injuries. It would appear that patients on melatonin supplements should continue taking them perioperatively because there may be benefits. Melatonin and its analogues will be increasingly encountered in the perioperative setting.

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Endogenous melatonin and oxidatively damaged guanine in DNA

BACKGROUND

A significant body of literature indicates that melatonin, a hormone primarily produced nocturnally by the pineal gland, is an important scavenger of hydroxyl radicals and other reactive oxygen species. Melatonin may also lower the rate of DNA base damage resulting from hydroxyl radical attack and increase the rate of repair of that damage. This paper reports the results of a study relating the level of overnight melatonin production to the overnight excretion of the two primary urinary metabolites of the repair of oxidatively damaged guanine in DNA.

METHODS

Mother-father-daughter(s) families (n = 55) were recruited and provided complete overnight urine samples. Total overnight creatinine-adjusted 6-sulphatoxymelatonin (aMT6s/Cr) has been shown to be highly correlated with total overnight melatonin production. Urinary 8-oxo-7,8-dihydro-guanine (8-oxoGua) results from the repair of DNA or RNA guanine via the nucleobase excision repair pathway, while urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) may possibly result from the repair of DNA guanine via the nucleotide excision repair pathway. Total overnight urinary levels of 8-oxodG and 8-oxoGua are therefore a measure of total overnight guanine DNA damage. 8-oxodG and 8-oxoGua were measured using a high-performance liquid chromatography-electrospray ionization tandem mass spectrometry assay. The mother, father, and oldest sampled daughter were used for these analyses. Comparisons between the mothers, fathers, and daughters were calculated for aMT6s/Cr, 8-oxodG, and 8-oxoGua. Regression analyses of 8-oxodG and 8-oxoGua on aMT6s/Cr were conducted for mothers, fathers, and daughters separately, adjusting for age and BMI (or weight).

RESULTS

Among the mothers, age range 42-80, lower melatonin production (as measured by aMT6s/CR) was associated with significantly higher levels of 8-oxodG (p < 0.05), but not with 8-oxoGua. Among the fathers, age range 46-80, lower melatonin production was associated with marginally higher levels of 8-oxoGua (p < 0.07), but not with 8-oxodG. Among the daughters, no relationship was found between melatonin levels and either 8-oxodG or 8-oxoGua levels. When the mother and father data were further analyzed using only subjects older than the oldest daughter, the associations became somewhat stronger.

CONCLUSION

Low levels of endogenous melatonin production among older individuals may lead to higher levels of oxidatively damaged guanine in DNA, thereby possibly increasing the risk of developing cancer. The possible different effects of melatonin in the rates of utilization of pathways for repair of oxidatively damaged guanine in DNA identified between older women and older men are intriguing.

Melatonin in pathogenesis and therapy of cancer

Melatonin is a neuroendocrine hormone secreted by the pineal gland to transduce the body's circadian rhythms. An internal 24 hour time keeping system (biological clock) regulated by melatonin, controls the sleep-wake cycle. Melatonin production is a highly conserved evolutionary phenomenon. The indole hormone is synthesized in the pinealocytes derived from photoreceptors. Altered patterns and/or levels of melatonin secretion have been reported to coincide with sleep disorders, jetlag, depression, stress, reproductive activities, some forms of cancer and immunological disorders. Lately, the physiological and pathological role of melatonin has become a priority area of investigation, particularly in breast cancer, melanoma, colon cancer, lung cancer and leukemia. According to the 'melatonin hypothesis' of cancer, the exposure to light at night (LAN) and anthropogenic electric and magnetic fields (EMFs) is related to the increased incidence of breast cancer and childhood leukaemia via melatonin disruption. Melatonin's hypothermic, antioxidant and free radical scavenging properties, attribute it to an immunomodulator and an oncostatic agent as well. Many clinical studies have envisaged the potential therapeutic role of melatonin in various pathophysiological disorders, particularly cancer. A substantial reduction in risk of death and low adverse effects were reported from various randomized controlled trials of melatonin treatment in cancer patients. This review summarizes the physiological significance of melatonin and its potential role in cancer therapy. Furthermore, the article focuses on melatonin hypothesis to represent the cause-effect relationship of the three aspects: EMF, LAN and cancer.

Acutely administered melatonin is beneficial while chronic melatonin treatment aggravates the evolution of TNBS-induced colitis

The aim of this study was to evaluate the effects of melatonin on the inflammatory response and hydroxyproline production in an experimental acute and chronic model of trinitrobenzene sulfonic (TNBS) acid-induced colitis in Wistar rats. In the acute model, melatonin (0.5, 1, and 2 mg/kg, i.p.) was applied 48, 24, and 1 hr prior to the induction of colitis and 24 and 48 hr after; the severity of colitis was less evident in melatonin-treated animals with significant response in the group treated with 2 mg/kg. All doses investigated significantly reduced the myeloperoxidase activity (MPO). In the chronic studies, melatonin (1 and 2 mg/kg, i.p.) was administered daily 24 hr before hapten instillation and for 7 or 21 days after TNBS; melatonin (2 mg/kg) worsened colitis evolution in the 21-day study with a significant increase in MPO activity and tumor necrosis factor-alpha production with respect to TNBS group. Histological slides were in concordance with macroscopic data where areas of extensive necrosis and edema, fibrosis, and absence of regenerated epithelium were observed. Moreover, the hydroxyproline determination, used as indicator of collagen production and fibrosis, also showed a marker increase. The results obtained in this experimental model showed that short-term administration is protective while in the long term it negatively influences evolution of inflammatory colitis; therefore, the immunostimulatory effect of melatonin in some situations when given chronically, such as during inflammatory bowel disease, might lead to negative consequences.

Protective effect of melatonin against fractionated irradiation-induced epiphyseal injury in a weanling rat model

The effects of melatonin, a free-radical scavenger and a general antioxidant, on radiation-induced growth plate injury have not been studied previously. The purpose of this study was to determine the potential benefits of sparing longitudinal bone growth by fractionated radiotherapy alone compared with pretreatment with melatonin that provides differential radioprotection of normal cells. Weanling 4-wk-old (75-100 g) male Sprague-Dawley rats were randomly assigned to one of three groups: Group R received fractionated radiation alone (n = 8); groups M5 (n = 8) and M15 (n = 7) received 5 or 15 mg/kg melatonin prior to fractionated radiation, respectively. The distal femur and proximal tibia in the right leg of each animal were exposed to a therapeutic X-irradiation dose (25 Gy total in three fractions) with the contralateral left leg as the non-irradiated control. Melatonin was administered intraperitoneally to the animals 30 min before radiation exposure. Six weeks after treatment, the rats were killed and the lower limbs disarticulated, skeletonized, radiographed, and bone growth was calculated based on measurement of the bone lengths. Fractionated radiation resulted in a mean percent overall limb growth loss of 41.2 +/- 9.5 and a mean percent overall limb discrepancy of 11.2 +/- 2.2. The administration of 5 or 15 mg/kg melatonin before each of the three fractions of radiotherapy reduced the mean percent overall limb growth loss to 33.9 +/- 5.8 and 32.2 +/- 4.5, respectively, and the mean percent overall limb discrepancy to 9.4 +/- 1.6 and 8.9 +/- 1.1, respectively; these values were significantly different compared with irradiation alone (range: P = 0.01-0.04). When compared with Group R, the growth arrest recovered by 5 or 15 mg/kg melatonin was 19.7 and 24.1% for the tibia, 7 and 18.6% for the femur, and 17.7 and 21.8% for the total limb, respectively. These results support further investigation of melatonin in combination with fractionation for potential use in growing children requiring radiotherapy to the extremity for malignant tumors.

Characterization of the protective effects of melatonin and related indoles against alpha-naphthylisothiocyanate-induced liver injury in rats

The protective effect of melatonin, 6-hydroxymelatonin and N-acetylserotonin against alpha-naphthylisothiocyanate (ANIT)-induced liver injury was investigated and compared in rats injected once with the hepatotoxicant (75 mg/kg body weight). In rats injected with ANIT alone, liver injury with cholestasis developed within 24 h, as indicated by both serum levels of alanine aminotransferase (SGPT) and aspartic acid aminotransferase (SGOT) activities and serum total bilirubin concentration. The administration of melatonin or 6-hydroxymelatonin (10 mg/kg body weight) to ANIT-injected rats reduced significantly the serum levels of both SGPT and SGOT and the serum total bilirubin concentration. For all hepatic biochemical markers, melatonin was more effective that 6-hydroxymelatonin. By comparison, the administration of N-acetylserotonin (10 mg/kg body weight) to ANIT-injected rats did not reduce the serum levels of either hepatic enzymes or the serum total bilirubin concentration. In ANIT-injected rats, hepatic lipid peroxidation (LPO) was significantly higher than in control animals and this increase was significantly reduced by either melatonin, 6-hydroxymelatonin or N-acetylserotonin. Furthermore, ANIT treatment caused a significant reduction in liver microsomal membrane fluidity and this reduction was completely reversed by the three indoles. The liver from ANIT-injected rats showed several histopathological alterations; above all there was an acute infiltration of polymorphonuclear neutrophils and an increase in the number of apparent apoptotic hepatocytes. The concurrent administration of melatonin reduced the severity of all morphological alterations, specially the neutrophil infiltration and the number of presumed apoptotic cells. On the contrary, the administration of 6-hydroxymelatonin or N-acetylserotonin did not provide any protective effect in terms of the histopathological alterations. These results indicate that melatonin protects against ANIT-induced liver injury with cholestasis in rats, and suggests that this protective effect is likely due to its antioxidant properties and above all to its capacity to inhibit liver neutrophil infiltration, a critical factor in the pathogenesis of ANIT-induced liver injury. 6-hydroxymelatonin, although able to provide partial protection against the ANIT-induced hepatic injury, probably through its antioxidant properties by mechanisms that are unclear, was unable to reduce neutrophil infiltration. Finally, N-acetylserotonin in the experimental conditions of this study, only exhibited some antioxidant protection but had no protective effect against ANIT-induced hepatic damage.

Ototoxicity caused by aminoglycosides is ameliorated by melatonin without interfering with the antibiotic capacity of the drugs

The production of free radicals seems to be involved in the mechanisms of ototoxicity. Aminoglycosides produce ototoxicity, which can be determined through distortion product otoacoustic emissions (OAEs) that measure the activity of the outer hair cells of the organ of Corti. An ototoxic chart was obtained in rats using gentamicin or tobramycin. Together with this treatment, the animals ingested melatonin in the drinking water, or melatonin was injected subcutaneously or intramuscularly. The distortion product OAEs were determined over a prolonged period of time for each of the groups. The effect of melatonin on the antibiotic capacity of the aminoglycosides used was also studied. Antibiograms inoculated with Escherichia coli or Pseudomonas aeruginosa and treated with gentamicin or tobramycin in the presence or absence of melatonin at quantities from pharmacological to physiological doses were performed. The ototoxicity produced by gentamicin and tobramycin was maximal from days 3 to 5 post-treatment, returning to normal values in 2 wk. When melatonin was present, the recovery was at day 5 post-treatment, independently of the means of administration of the pineal product. The antibiograms showed that melatonin had no effect on the antibiotic capacity. It is concluded that the ototoxicity caused by gentamicin and tobramycin is ameliorated by melatonin and that the pineal hormone does not interfere with the antibiotic capacity of these antibiotics.

Melatonin and other antioxidants prolong the postmortem activity of the outer hair cells of the organ of Corti: its relation to the type of death

The outer hair cells of the organ of Corti transform sound into electrical signals, beginning the nervous auditive process. These cells produce acoustic emissions when working routinely, known as otoacoustic emissions. Otoacoustic emissions (OAEs) are recorded from the hearing duct through a probe which incorporates a sound source and a sensitive microphone. On the other hand, the cochlea produces oxygen-derived free radicals and nitric oxide, in addition, melatonin is present in the cochlea. The authors have studied the influence of melatonin or an antioxidant mixture (alpha-tocopherol acid succinate, ascorbic acid, glutathione, and N-acetylcysteine) on the postmortem activity of the outer hair cells of the organ of Corti of the rat, measuring distortion product otoacoustic emissions. Control rats showed postmortem distortion product otoacoustic emissions for about 2 min when sacrificed by decapitation, and for about 3 min when sacrificed by chloroform inhalation. Melatonin prolonged the postmortem activity 3.5 times when the animals were sacrificed by decapitation, and 7 times when animals were sacrificed by chloroform inhalation. Similar results were obtained with the antioxidant mixture. Results show that melatonin and other antioxidants have, in general, a protective role on the postmortem activity of the outer hair cells of the organ of Corti.

Regulation of the expression of serotonin N-acetyltransferase gene in Japanese quail (Coturnix japonica): I. Rhythmic pattern and effect of light

Serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AANAT); EC2.3.1.87] is the rate-limiting enzyme in melatonin synthesis, and its activity exhibits a diurnal rhythm similar to that of the melatonin content in the pineal gland and retina of Japanese quail. Studies were conducted to characterize the Japanese quail AANAT cDNA, and to evaluate the expression of AANAT mRNA in the pineal gland, the retina, and other peripheral tissues. The nucleic acid sequence of a 400 bp cDNA clone obtained by RT-PCR manifested 78 and 95% homology compared to the rat and chicken AANAT cDNA, respectively, while the deduced amino acid sequence homology was 82 and 99%, respectively. AANAT mRNA content in a single pineal gland or an aliquot of eye lysate was measured by a micro-lysate protection assay. The expression of AANAT mRNA in the pineal gland and the retina exhibited circadian rhythm with peak levels at night. AANAT mRNA was also detected in the testis, but did not display a rhythmic change over a 24 hr period. AANAT mRNA was not detected in other tissues studied. Darkness during the day did not increase the pineal AANAT mRNA levels. However, unexpected light-exposure for 2 hr just after lights-off blocked the increase in AANAT mRNA, and at midnight remarkably decreased AANAT mRNA by 50%.

Regulation of the expression of serotonin N-acetyltransferase gene in Japanese quail (Coturnix japonica): II. Effect of vitamin A deficiency

Levels of serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AANAT); EC2.3.1.87] mRNA, AANAT activity, and melatonin display a rhythmic pattern in both the pineal gland and the retina. It has been shown that vitamin A is required to maintain the rhythm of melatonin synthesis in the pineal gland of Japanese quail. To understand the mechanism underlying the direct relationship among these factors, we developed an assay system sensitive enough to determine AANAT mRNA, AANAT activity, and melatonin content from a single pineal gland of Japanese quail. Positive direct relationships were found among these three parameters. We next deprived Japanese quail of vitamin A by feeding them a vitamin A-free diet supplemented with retinoic acid, and examined the effects of vitamin A deficiency on the expression of AANAT mRNA in the pineal gland and the retina. Vitamin A deficiency reduced both the expression of AANAT mRNA and melatonin content in the pineal gland. Retinal AANAT mRNA rhythm disappeared in vitamin A-deficient quails. Moreover, the responsiveness of the pineal gland and the retina to light was reduced by vitamin A deficiency when compared with the control group.

Scavenging of reactive oxygen species by melatonin

The direct effects of the neurohormone melatonin on reactive oxygen species (ROS) were investigated. Melatonin was found to inhibit DMPO-O-2 formation in a dose-dependent manner. At the level of 1. 7+/-0.07 mM, melatonin caused 50% inhibition of EPR signal intensity of DMPO-O-2 during the reaction of xanthine and xanthine oxidase. The reaction rate constant of melatonin with O2- was found to be 1.25+/-0.07x103 M-1 s-1. However, melatonin (up to 1.2 mM) did not exhibit significant effect toward OH radical, produced by the Fenton reaction. In addition, we found no evidence for the formation of the melatonin indolyl cation radical that presumably precedes conversion of melatonin to its stable N1-acetyl-N2-5-methoxykynuramine (AMK) metabolite following sequential reactions of melatonin with O2- and OH. On the other hand, melatonin was capable of scavenging H2O2 in a dose-dependent manner with an IC50=0.5+/-0.02 mM. The reaction rate constant of melatonin with H2O2 was found to be 2.52+/-0.19x105 M-1 s-1. Furthermore, melatonin was also found to inhibit 1O2-dependent 2,2,6,6-tetramethylpiperidine oxide (TEMPO) radical formation during rose bengal photodynamic reaction. The results suggest that melatonin's antioxidant properties, in part, may involve a direct effect on scavenging of ROS.