Neuroscience Letters 518 (2012) 32–35 Contents lists available at SciVerse ScienceDirect Neuroscience Letters j our nal ho me p ag e: www.elsevier.com/locate/neulet Circadian modulation of human erythrocyte plasma membrane redox system by melatonin Shilpa Chakravarty, Syed Ibrahim Rizvi ∗ Department of Biochemistry, University of Allahabad, Allahabad 211002, India a r t i c l e i n f o Article history: Received 16 January 2012 Received in revised form 14 April 2012 Accepted 17 April 2012 Keywords: Erythrocyte Melatonin Plasma membrane redox system Ascorbate free radical reductase a b s t r a c t Background: The pineal hormone melatonin works for the stabilization of biological rhythms, however, it also modulates several other functions such as cardioprotection, thermoregulation and immunomodula- tion. Melatonin also shows antioxidant activity. The erythrocyte plasma membrane redox system (PMRS) alongwith ascorbate free radical (AFR) reductase is involved in providing protection against oxidative stress. The present work is an ex vivo study addressing RBC PMRS and AFR reductase activities at two dif- ferent times of the day. The in vitro modulatory effect of melatonin on PMRS and AFR reductase activities is also reported. Materials and methods: The study was carried out on 61 healthy donors of both sexes (aged 20–30). Blood samples were collected at two different timings viz., 10:00 a.m. and 10:00 p.m. PMRS and AFR reductase were determined by methods already reported. The concentration-dependent effect of the melatonin was evaluated by incubating the RBCs with the hormone at different doses. Results: We present results to show that erythrocyte PMRS and AFR reductase activity are modulated by melatonin, a higher activity (p < 0.05) of PMRS and AFR reductase is observed during night when the level of melatonin is high. The effect of in vitro treatment of erythrocytes with melatonin (10 -7 M to 10 -11 M final concentration) shows significant changes during day at a melatonin concentration of 10 -9 M. Conclusion: To the best of our knowledge this study shows for the first time the circadian rhythmicity of erythrocyte PMRS and AFR reductase activities. The modulatory effect of melatonin on PMRS and AFR reductase opens the possibility of melatonin being used in treatment of such physiological and metabolic dysfunctions that involve photic cues in association with oxidative stress. © 2012 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Melatonin is secreted by the pineal gland in the brain. In all species, regardless of their activity pattern, i.e., whether they are diurnal, nocturnal or crepuscular, pineal melatonin is produced pri- marily during the dark phase of the circadian cycle. In humans, the nocturnal secretion of melatonin reaches its peak value of 0.3 nM in plasma between 11:00 p.m. and 3.00 a.m. Previously melatonin was believed to synchronize seasonal breeding in photoperiodic species; however, during the last two decades several publica- tions have documented the involvement of melatonin in many other functions such as cardioprotection, immunomodulation, thermoregulation, gastrointestinal regulation, tumor-suppression, neuroprotection as well as anti-aging besides the entrainment of daily sleep/wake rhythm [19]. It is already established that melatonin is a direct free radical scavenger and can stimulate several antioxidative enzymes. There ∗ Corresponding author. Tel.: +91 9415305910; fax: +91 532 2242116. E-mail address: sirizvi@gmail.com (S.I. Rizvi). are reports which emphasize the efficiency of melatonin against free radical-induced cellular damage and biochemical impairments [22]. It has been found to be more efficient in reducing oxidative stress than other antioxidants such as glutathione and mannitol [20]. Eukaryotic cells display a plasma membrane redox system (PMRS) that transfers electrons from intracellular substrates to extracellular electron acceptors. The PMRS incorporates ascorbate free radical (AFR) reductase which is involved in regeneration of extracellular ascorbate by transferring reducing equivalents from intracellular ascorbate, a mechanism which plays an important role in maintaining plasma ascorbate levels [13]. Although the exact physiological function of this PMRS remains elusive, proposed func- tions include: maintenance of redox state of sulfhydryl residues in membrane proteins, neutralization of oxidative stressors out- side the cells, stimulation of cell growth, recycling of ˛-tocopherol, reduction of lipid hydroperoxides, reduction of ferric ions prior to iron uptake by a transferring-independent pathway, and the maintenance of the extracellular concentration of ascorbic acid [14,16,15,28]. The red blood cell (RBC) is constantly exposed to oxidative stress; protection against the ROS generated during the 0304-3940/$ – see front matter © 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.neulet.2012.04.042