Diabetes mellitus has been shown to be a state of increased free radicals and
disturbance in antioxidant defense system. The aim of the present study was to
investigate the protective effect of melatonin on some oxidative stress parameters in
both blood and tissues, liver and pancreas, in streptozotocin (STZ)-induced diabetes
in rats. Three groups of rats were used, control group, diabetic group and a group of
diabetic rats treated with melatonin. Malondialdehyde (MDA) level was used as a
marker of lipid peroxidation, advanced oxidation protein product (AOPP) level as a
marker for protein damage and activity of antioxidant enzymes; glutathione
peroxidase (GPx) and paraoxonase (PON) were, also, assessed. Liver and
pancreatic malondialdehyde and plasma AOPP levels were significantly reduced but
serum PON1 and pancreatic GPx activities were significantly increased by melatonin
treatment. Liver GPx activities showed no significant differences among all studied
groups. Microscopic examination of liver showed improvement of the morphological
and histopathological changes induced by diabetes. These data suggested that
melatonin treatment has a partial therapeutic effect in diabetes by reduction of
oxidative stress and by stimulation of various antioxidant enzymes.