Melatonin reduces gamma radiation-induced primary DNA damage in human blood lymphocytes

Peripheral blood samples were collected from human volunteers 5-10 min before, and at 1 and 2 h after a single oral dose of 300 mg of melatonin. At each time point: (1) the concentration of melatonin in the serum and in the leukocytes was determined; and (2) the whole blood was exposed in vitro to 100 cGy of gamma radiation. Immediately after exposure to the radiation, the lymphocytes were examined to determine the extent of primary DNA damage, viz., single strand breaks and alkali labile lesions (determined from the length of DNA migration and fluorescence intensity of migrated DNA in the comet tail), using the alkaline comet assay. For each volunteer, the results showed a significant increase in the concentration of melatonin in the serum and in the leukocytes at 1 h after the oral dose of melatonin, as compared to the sample collected at 0 hour. The lymphocytes in the blood samples collected at 1 and 2 h after melatonin ingestion and exposed in vitro to 100 cGy gamma radiation exhibited a significant decrease in the extent of primary DNA damage, as compared with similarly irradiated lymphocytes from the blood sample collected before melatonin ingestion. The extent of the melatonin-associated decrease in primary DNA damage did not correspond with the decrease reported earlier in the incidence of chromosomal aberrations and micronuclei; the latter assays required an additional postirradiation incubation of the cells at 37 +/- 1 degrees C for 48 and 72 h, respectively.