Melatonin: a well-documented antioxidant with conditional pro-oxidant actions

Authors

  • Hong-Mei Zhang,

    Corresponding author
    1. Department of Clinical Oncology, Xijing Hospital, The Fourth Military Medical University, Xi'an, China
    • Address reprint requests to Hong-Mei Zhang, Department of Oncology, Xijing Hospital, The Fourth Military Medical University, Xi'an 710032, China.

      E-mail: zhm@fmmu.edu.cn

      and

      Yiqiang Zhang, Barshop Institute for Longevity and Aging Studies & Department of Physiology, The University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, San Antonio, TX 78245, USA.

      E-mail: zhangy@uthscsa.edu

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  • Yiqiang Zhang

    Corresponding author
    1. Barshop Institute, The University of Texas Health Science Center at San Antonio, San Antonio, TX, USA
    2. Department of Physiology, The University of Texas Health Science Center at San Antonio, San Antonio, TX, USA
    • Address reprint requests to Hong-Mei Zhang, Department of Oncology, Xijing Hospital, The Fourth Military Medical University, Xi'an 710032, China.

      E-mail: zhm@fmmu.edu.cn

      and

      Yiqiang Zhang, Barshop Institute for Longevity and Aging Studies & Department of Physiology, The University of Texas Health Science Center at San Antonio, 15355 Lambda Drive, San Antonio, TX 78245, USA.

      E-mail: zhangy@uthscsa.edu

    Search for more papers by this author

Abstract

Melatonin (N-acetyl-5-methoxytryptamine), an indoleamine produced in many organs including the pineal gland, was initially characterized as a hormone primarily involved in circadian regulation of physiological and neuroendocrine function. Subsequent studies found that melatonin and its metabolic derivatives possess strong free radical scavenging properties. These metabolites are potent antioxidants against both ROS (reactive oxygen species) and RNS (reactive nitrogen species). The mechanisms by which melatonin and its metabolites protect against free radicals and oxidative stress include direct scavenging of radicals and radical products, induction of the expression of antioxidant enzymes, reduction of the activation of pro-oxidant enzymes, and maintenance of mitochondrial homeostasis. In both in vitro and in vivo studies, melatonin has been shown to reduce oxidative damage to lipids, proteins and DNA under a very wide set of conditions where toxic derivatives of oxygen are known to be produced. Although the vast majority of studies proved the antioxidant capacity of melatonin and its derivatives, a few studies using cultured cells found that melatonin promoted the generation of ROS at pharmacological concentrations (μm to mm range) in several tumor and nontumor cells; thus, melatonin functioned as a conditional pro-oxidant. Mechanistically, melatonin may stimulate ROS production through its interaction with calmodulin. Also, melatonin may interact with mitochondrial complex III or mitochondrial transition pore to promote ROS production. Whether melatonin functions as a pro-oxidant under in vivo conditions is not well documented; thus, whether the reported in vitro pro-oxidant actions come into play in live organisms remains to be established.

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