Comparison of the inactivation of mammalian and bacterial O6-alkylguanine-DNA alkyltransferases by O6-benzylguanine and O6-methylguanine

Abstract

The inactivation of human and Escherichia coli O⁶-alkylguanine-DNA alkyltransferase by O⁶-methylguanine and O⁶-benzylguanine was compared. When HT29 cell extracts or E.coli Ada protein were incubated in the presence of 200 μM O⁶-methylguanine for 1 h, alkyltransferase activity was reduced to 44 and 39% of control levels respectively. However, under the same conditions O⁶-benzylguanine completely depleted alkyltransferase activity in the extract from human cells but had virtually no effect on the Ada protein. Incubation of the HT29 cell alkyltransferase with O⁶-benzyl[³H]guanine resulted in a time-dependent production of [3^H]guanine. No similar production of [3^H]guanine was observed in the presence of the Ada protein. In CHO cells transfected with the bacterial ada gene (CHO-ada) or the human alkyltransferase cDNA (CHO-MGMT), treatment with 500 μM O⁶-methylguanine inhibited both alkyltransferases by >85%. In contrast, 2 μM O⁶-benzylguanine inhibited human alkyltransferase expressed in CHO-MGMT cells by >99% though concentrations as high as 25 μM for 24 h had no inhibitory effects on the bacterial akyltransferase expressed in CHO-ada cells. This selective inhibition was also observed in vivo in transgenic mice expressing ada in the liver where O⁶-benzylguanine caused a decrease of only 40% in total hepatic alkyltransferase activity compared to 95% in non-transgenic mice, consistent with inhibition of only the mammalian alkyltransferase and maintenance of bacterial alkyltransferase activity in these animals. Thus, while O⁶-methylguanine at high concentrations inactivates both bacterial and mammalian alkyltransferases, O⁶-benzylguanine is a substrate only for the mammalian protein and is unable, perhaps due to steric hindrance, to inhibit the Ada protein.