Antioxidant properties of melatonin: a pulse radiolysis study

Mahal, H. S.; Sharma, H. S.; Mukherjee, Tulsi

doi:10.1016/s0891-5849(98)00226-3

Cited by 164 publications

(143 citation statements)

References 33 publications

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“…A variety of methodologies including nitrogencontaining trapping agents, salicylate metabolites, high-performance liquid chromatography (HPLC), electron spin resonance (ESR) spectroscopy, exploratory laser flash photolysis and pulse radiolysis, have all been used to clarify melatonin's ability to neutralize the HO W . All these methods proved that melatonin is a powerful HO W scavenger [17,[33][34][35][36][37][38][39][40][41][42][43]. Most recently, using a new trapping agent, 5-diethoxyphosphoryl-5-methyl-1-pyrroline-Noxide (DEPMPO), and ESR spectroscopy, it was again confirmed that melatonin is a potent HO W scavenger [44,45] (fig.…”

Section: Melatoninmentioning

confidence: 96%

“…Based on these observations, it was suggested that the indolyl-type radical is the main product of melatonin interaction with HO W . More recently, Mahal et al [43] also found evidence for the existence of melatonyl radical cation in a cell-free system using the methods of pulse radiolysis combined with a spectrophotometric monitoring system. In this study, melatonin was considered as an excellent electron donor.…”

Section: Proposed Mechanisms Of Melatonin Scavenging Free Radicalsmentioning

confidence: 99%

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Significance of Melatonin in Antioxidative Defense System: Reactions and Products

et al. 2000

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Melatonin is a potent endogenous free radical scavenger, actions that are independent of its many receptor-mediated effects. In the last several years, hundreds of publications have confirmed that melatonin is a broad-spectrum antioxidant. Melatonin has been reported to scavenge hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), nitric oxide (NO·), peroxynitrite anion (ONOO^–), hypochlorous acid (HOCl), singlet oxygen (¹O₂), superoxide anion (O₂^–·) and peroxyl radical (LOO·), although the validity of its ability to scavenge O₂^–· and LOO· is debatable. Regardless of the radicals scavenged, melatonin prevents oxidative damage at the level of cells, tissues, organs and organisms. The antioxidative mechanisms of melatonin seem different from classical antioxidants such as vitamin C, vitamin E and glutathione. As electron donors, classical antioxidants undergo redox cycling; thus, they have the potential to promote oxidation as well as prevent it. Melatonin, as an electron-rich molecule, may interact with free radicals via an additive reaction to form several stable end-products which are excreted in the urine. Melatonin does not undergo redox cycling and, thus, does not promote oxidation as shown under a variety of experimental conditions. From this point of view, melatonin can be considered a suicidal or terminal antioxidant which distinguishes it from the opportunistic antioxidants. Interestingly, the ability of melatonin to scavenge free radicals is not in a ratio of mole to mole. Indeed, one melatonin molecule scavenges two HO·. Also, its secondary and tertiary metabolites, for example, N¹-acetyl-N²-formyl-5-methoxykynuramine, N-acetyl-5-methoxykynuramine and 6-hydroxymelatonin, which are believed to be generated when melatonin interacts with free radicals, are also regarded as effective free radical scavengers. The continuous free radical scavenging potential of the original molecule (melatonin) and its metabolites may be defined as a scavenging cascade reaction. Melatonin also synergizes with vitamin C, vitamin E and glutathione in the scavenging of free radicals. Melatonin has been detected in vegetables, fruits and a variety of herbs. In some plants, especially in flowers and seeds (the reproductive organs which are most vulnerable to oxidative insults), melatonin concentrations are several orders of magnitude higher than measured in the blood of vertebrates. Melatonin in plants not only provides an alternative exogenous source of melatonin for herbivores but also suggests that melatonin may be an important antioxidant in plants which protects them from a hostile environment that includes extreme heat, cold and pollution, all of which generate free radicals.

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Section: Melatoninmentioning

confidence: 96%

Section: Proposed Mechanisms Of Melatonin Scavenging Free Radicalsmentioning

confidence: 99%

Significance of Melatonin in Antioxidative Defense System: Reactions and Products

et al. 2000

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“…10 10 Chyan et al [20] ultraviolet photolysis of H 2 O 2 kinetic competition with ABTS or DMPO 4.0 ! 10 10 Mahal et al [52] pulse radiolysis of water absorption spectra of indolyl radical 1.25 ! 10 10…”

Section: Studies Where the Rate Constant Was Determinedmentioning

confidence: 99%

“…There are a number of radical intermediates between the indolyl radical and cyclic 3-hydroxymelatonin; the details of the complete pathway are available in the original paper by Tan et al [104]. Mahal et al [52] claim that melatonin may be regenerated from the indolyl radical by ascorbate or urate. Melatonin may also scavenge 1 O 2 and H 2 O 2 with the production of N 1 -acetyl-N 2 -formyl-5-methoxykynuramine (AFMK) which is then enzymatically converted, by catalase, to N 1 -acetyl-5-methoxykynuramine.…”

Section: Studies Where the Rate Constant Was Determinedmentioning

confidence: 99%

“…There is also some evidence that the indolyl radical is recycled back to melatonin. Whereas Mahal et al [52] suggested that either ascorbate or urate is capable of recycling the indolyl radical, the details of these potential interactions are only scanty and as yet unconfirmed. If in fact melatonin is regenerated, it could explain the synergistic actions of ascorbate and melatonin as described by Poeggeler et al [77].…”

Section: Studies Where the Rate Constant Was Determinedmentioning

confidence: 99%

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Actions of Melatonin in the Reduction of Oxidative Stress

et al. 2000

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Melatonin was discovered to be a direct free radical scavenger less than 10 years ago. Besides its ability to directly neutralize a number of free radicals and reactive oxygen and nitrogen species, it stimulates several antioxidative enzymes which increase its efficiency as an antioxidant. In terms of direct free radical scavenging, melatonin interacts with the highly toxic hydroxyl radical with a rate constant equivalent to that of other highly efficient hydroxyl radical scavengers. Additionally, melatonin reportedly neutralizes hydrogen peroxide, singlet oxygen, peroxynitrite anion, nitric oxide and hypochlorous acid. The following antioxidative enzymes are also stimulated by melatonin: superoxide dismutase, glutathione peroxidase and glutathione reductase. Melatonin has been widely used as a protective agent against a wide variety of processes and agents that damage tissues via free radical mechanisms.

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Reactivity of Peroxynitrite with Melatonin as a Function of pH and CO2 Content

et al. 2003

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Peroxynitrite is known to be a strong oxidant and a nitrating agent of aromatic phenolic or heterocyclic rings, depending on its form in aqueous solutions: the anion ONOO − , its conjugate acid ONOOH, or its CO 2 adduct ONOOCO 2 − . Various reactions have been observed with melatonin (1), a tryptophan derivative, in phosphate-buffered solutions. Melatonin (1) is recognized as a scavenger of several strong oxidants (HO · , H 2 O 2 , ...) accounting for its biological and pharmacological effects. Here we describe two oxidation routes that give rise to indol-2-ones 2 (probably via a 2,3-epoxyindole) and kynuramines 6 (by cleavage of the pyrrole ring), attributable to reactions of ONOOH and ONOO − , respectively, according to the effects of pH and CO 2 content. At pH = 7.6 and in the presence of CO 2 , an important conversion is the cyclization of the lateral amide function, giving 3-substituted pyrroloindoles 4. At neutral pH, therefore, all routes coexist, with a balance between indol-2-ones 2 and pyrroloindoles 4 on the one side and kynuramines 6 on the other, depending on the

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Antioxidant properties of melatonin: a pulse radiolysis study

Cited by 164 publications

References 33 publications

Significance of Melatonin in Antioxidative Defense System: Reactions and Products

Significance of Melatonin in Antioxidative Defense System: Reactions and Products

Actions of Melatonin in the Reduction of Oxidative Stress

Reactivity of Peroxynitrite with Melatonin as a Function of pH and CO2 Content

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