Crude extracts of human KB cells grown in suspension culture contain enzyme activity that catalyzes the preferential excision of thymine-containing pyrimidine dimers from UV-irradiated E. coli DNA specifically incised adjacent to dimer sites. Fractionation of KB cell crude extracts reveals the presence of three such activities with distinct affinities for both DEAE-cellulose and phosphocellulose. One of the activities (activity B) is distinguished by its S20,w (2.6) and iso electric point (9.0) from the other two (activities A and C) which have similar S20.w's (3.0-3.2) and isoelectric points (6.0). All three differ in their extent of stimulation by divalent cation and inhibition by NaCl or a sulfhydryl group inhibitor. These results indicate that multiple 5' →3' dimer excision nuclease activities exist in human cells; however, there is as yet no direct evidence that these enzymes are functional in nucleotide excision repair in vivo.
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