DNA photoreactivating enzyme in human leukocytes is concentrated in the phagocytotic monocytes and polymorphonuclear cells. Lymphocytes, erythrocytes, spleen, and serum contain little if any enzyme activity. Since bone marrow, which contains immature, nonphagocytotic monocytes and polymorphonuclear cells, also contains high levels of enzyme, it is unlikely that the enzyme in the mature cells resulted from bacteria engulfed by these cells. Photoreactivating enzyme is also found in murine, and in human cells in culture; confluent murine cells have higher specific activity than do rapidly growing cells. The leukocyte enzyme, which requires ultraviolet-irradiated DNA as substrate and visible light for catalysis, converts pyrimidine dimers in DNA to the corresponding monomers in the lightdependent reaction. The action spectrum for photoreactivation extends from 300 to 600 nm, with a peak at about 400 nm.
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