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The aim of this study is to examine possible in vitro antioxidant effects of melatonin. Thus, the total in vitro antioxidant activity of melatonin was studied using a thiocyanate method. Additionally, the reducing power, the superoxide anion scavenging activity and free radical scavenging activity of melatonin were determined. Melatonin exhibited potent antioxidant activity in a linoleic acid emulsion system. The antioxidant activity increased with increasing concentrations of melatonin (50-500 microg). The 50, 100, 250 and 500 microg melatonin doses showed 41, 60, 86 and 99% inhibition of peroxidation of linoleic acid, respectively. On the other hand, a 500-microg dose of alpha-tocopherol showed 34% inhibition of peroxidation of linoleic acid. Like the total antioxidant activity, the reducing power of melatonin increased in a dose-dependent manner. The reducing power of melatonin was statistically significant versus control, but lower than butylated hydroxytoluene (BHT) or quercetin. Additionally, melatonin had potent superoxide radical scavenging activity and exhibited a higher superoxide radical scavenging activity than quercetin or BHT but lower than butylated hydroxyanisole (BHA). Melatonin's direct free radical scavenging actions may account, at least in part, for its ability to reduce lipid peroxidation. Melatonin may have utility in protecting stored foods from free radical-induced deterioration.

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:  Radiation therapy is a popular and useful tool in the treatment of cancer. Melatonin participates in the regulation of a number of important physiological and pathological processes. Melatonin, a powerful endogenous antioxidant, plays a role in the reduction of oxidative damage. Thirty adult rats were divided into five equal groups. On the day of the experiment, groups I and II were injected with 5 or 10 mg/kg melatonin, respectively, while group III received isotonic NaCl solution. Thirty minutes later, groups I, II and III were exposed to 6.0 Gy whole body ionizing radiation in a single fraction. Group IV was injected with 5 mg/kg melatonin but was not irradiated. The final group was reserved as sham treated. Liver malondialdehyde (MDA) and nitric oxide (NOċ) levels were measured in all groups. Whole body irradiation caused a significant increase in liver MDA and NOċ levels. Hepatic MDA and NOċ levels in irradiated rats that were pretreated with melatonin (5 or 10 mg/kg) were significantly decreased. Malondialdehyde and NOċ levels were reduced in a dose‐related manner by melatonin. The data show that melatonin reduces liver damage inflicted by irradiation when given prior to the exposure to ionizing radiation. The radioprotective effect of melatonin is likely achieved by its ability to function as a scavenger for free radicals generated by ionizing radiation.

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: Antioxidant activity of a molecule is attributed to various mechanisms such as prevention of chain initiation, binding of transition metal ion catalysts and decomposition of peroxides. This study was aimed at evaluating the metal chelating and hydrogen peroxide (H2O2) scavenging activity of melatonin. The metal chelating and H2O2 scavenging activity increased with increasing concentrations of melatonin (20–60 μg/mL). α‐Tocopherol, butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT) were used as standards. Sixty micrograms per milliliter concentration of melatonin exhibited 95% chelating effect on ferrous ions and scavenged 83% of H2O2. On the other hand, the same concentration of α‐tocopherol, BHA, and BHT exhibited 58, 61, and 72% inhibition, respectively, of the formation of the Fe2+–ferrozine complex and scavenged 48, 20, and 23%, respectively, of H2O2. Based on these results, it is concluded that melatonin is an effective metal chelating agent and scavenger of H2O2. These properties may be major reasons for the melatonin's ability to inhibit lipid peroxidation.

Plants with antidiabetic activities provide important sources for the development of new drugs in the treatment of diabetes mellitus. In the present study, we investigated possible antihyperglycemic and antioxidative activities of the aerial part of the Matricaria chamomilla L. ethanolic extract (MCE) in streptozotocin (STZ; 70 mg/kg, i.p.)-induced diabetic rats. The following groups were assigned; sham (did not receive any substance), STZ + distilled water (control), STZ + 5 mg/kg glibenclamide, STZ + 20 mg/kg MCE, STZ + 50 mg/kg MCE, STZ + 100 mg/kg MCE. Diabetic rats were treated for 14 days by gavage. Postprandial blood glucose levels, malondialdehyde, reduced glutathione (GSH), nitrate, nitrite, ascorbic acid, retinol, beta-carotene, superoxide dismutase, and catalase levels were measured, and immunohistochemical studies were performed in all of the groups. The obtained data showed that STZ resulted in oxidative stress and affected the antioxidant status. Treatment with different doses of MCE significantly reduced postprandial hyperglycemia and oxidative stress, and augmented the antioxidant system. In histological investigations, MCE treatment protected the majority of the pancreatic islet cells, with respect to the control group. As a result, MCE exhibited significant antihyperglycemic effect and protected beta-cells in STZ-diabetic rats, in a dose-dependent manner, and diminished the hyperglycemia-related oxidative stress.

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