The Human Leukocyte Photoreactivating Enzyme
A photoreactivating enzyme from human leukocytes has been isolated and characterized. The enzyme requires DNA irradiated with ultraviolet light (220–300 nm) as substrate, and visible light (300–600 nm) for catalysis. In the reaction, the enzyme converts cyclobutyl pyrimidine dimers in the DNA to monomer pyrimidines.
The enzyme has an apparent monomer molecular weight of 40,000 and tends to form aggregates. The pH optimum of 7.2 and absence of a requirement for metal ions are similar to the requirements of the Escherichia coli enzyme; however, the ionic strength optimum of 0.05 is much lower than those for other photoreactivating enzymes.
The demonstration that human cells possess photoreactivating enzyme implies that a direct test by photoreactivation may be made of the role of pyrimidine dimers in the induction of abnormal cell growth.
KeywordsHuman Leukocyte Placental Mammal Pyrimidine Dimer Rabbit Kidney Cell Dime Content
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