Human peripheral blood lymphocytes which were pretreated in vitro with melatonin, an endogenously synthesized pineal hormone, for 20 min at 37 ± 1°C exhibited a significant and concentration-dependent reduction in the frequency of γ-radiation-induced micronuclei compared with irradiated cells which did not receive the pretreatment. The extent of the reduction observed with 2.0 mM melatonin was similar to that found in lymphocytes pretreated for 20 min with 1.0 M dimethylsulfoxide, a known free radical scavenger. These observations indicate that melatonin may have an active role in protection of humans against genetic damage due to endogenously produced free radicals, and also may be of use in reducing damage due to exposure to physical and chemical mutagens and carcinogens which generate free radicals.

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