An enormous amount of data has been published in recent years demonstrating melatonin's defensive role against toxic free radicals. In the present study, we examined the role of melatonin as an antioxidant against the effect of continuous light exposure. Rats were divided into three groups. Control rats (group A) were kept under natural conditions whereas other group of rats (group B and C) were exposed to constant light for inhibition of melatonin secretion by the pineal gland. Group C rats also received melatonin via s.c. injection (1 mg x kg(- 1) body weight x day(- 1)). At the end of experiment, all animals were sacrificied by decapitation, serum and tissue samples were removed for determination of malondialdehyde (MDA), a product of lipid peroxidation, conjugated dienes levels and glutathione peroxidase (GSH-Px) activity levels. It was found that lipid peroxidation was increased in the rats which were exposed to constant light. Melatonin injection caused a decrease in lipid peroxidation, especially in the brain. In addition, melatonin application resulted in increased GSH-Px activity, which has an antioxidant effect. Thus, melatonin is not only a direct scavenger of toxic radicals, but also stimulates the antioxidative enzyme GSH-Px activity to detoxify hydroxyl radical produced by constant light exposure.

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The assay conditions needed to achieve maximal activity of liver and kidney arginase in diabetic and non-diabetic rats were investigated and compared. The physicochemical and kinetic properties of liver arginase in diabetic and control rats were very similar, those of kidney arginase were significantly different. It was found that preincubation temperature (68°C), preincubation period (20 min), optimum pH (10.1) of liver arginase and K_m (3.2) for its substrate, L-arginine, did not change in diabetic and non-diabetic rats. As a consequence of diabetes, the optimum Mn²⁺ concentration for liver arginase only changed from 1 to 2 mM. Although the preincubation temperature and period for activation of kidney arginase in control rats was unnecessary, they were found to be 56ºC and 12 min in diabetic rats. The pH profile of arginase in kidney of diabetic rats was different from that of control rats. The K_m value (6.7) of arginase for L-arginine in kidney is unchanged in diabetes whereas a marked decrease in V_max was found. Optimum Mn²⁺ concentration (2 mM) for kidney arginase was unchanged in diabetes. The activity of arginase in liver of diabetic animals was higher 1.5 to 1.7 times than that of controls. Diabetes caused an about 53% decrease of arginase activity in kidney of female rats, 26% in that of males. These findings may suggest an idea that encoded arginases by separate gene loci may be affected differently by the pathological and hormonal status.