O6-alkylguanine-DNA alkyltransferase activity in normal human tissues and cells

Normal adult human tissues and cultured bronchial epithelial cells and fibroblasts exhibit O6-alkylguanine-DNA alkyltransferase activity in vitro by catalyzing the repair of the promutagenic alkylation lesion O6-methylguanine from DNA. The amount repaired by extracts of liver, peripheral lung, and colon extracts was proportional to the amount of extract protein. Repair of O6-methylguanine led to stoichiometric regeneration of guanine in the DNA and stoichiometric formation of S-methylcysteine in protein. Alkyltransferase activity varies in the different human tissues tested in the decreasing order of liver greater than colon greater than esophagus greater than peripheral lung greater than brain. Extracts of lung tissues, cultured human bronchial epithelial cells, and fibroblasts had similar alkyltransferase activities. Various human tissues exhibit 2- to 10-fold higher alkyltransferase activity than corresponding rat tissues. Whereas the interindividual variation of the activity was 4- to 5-fold in ten or more human lung and colon specimens, the interindividual variation in the inbred rat was less than 20%. The present results show that different human tissues and cells have a several-fold higher capacity to repair O6-methylguanine in DNA than do rat tissues and that the repair process occurs via a mechanism similar to that shown previously in other mammalian cells and Escherichia coli.