Hydrolytic elimination of a mutagenic nucleotide, 8-oxodGTP, by human 18-kilodalton protein: sanitization of nucleotide pool

Abstract

8-Oxoguanine nucleotide can pair with cytosine and adenine nucleotides at almost equal efficiencies. Once 8-oxodGTP is formed in the cellular nucleotide pool, this mutagenic nucleotide is incorporated into DNA and would cause transversion mutations. The MutT protein of Escherichia coli possesses enzyme activity to hydrolyze 8-oxodGTP to the corresponding nucleoside monophosphate and thus may be responsible for preventing the occurrence of such mutations. Here we show that the human cell has an enzyme specifically hydrolyzing 8-oxodGTP in a fashion similar to that seen with MutT protein. The human 8-oxodGTPase has been found in cell-free extracts from Jurkat cells and purified > 400-fold. Analyses by gel filtration and gel electrophoresis revealed that the molecular mass of the native form of human 8-oxodGTPase is 18 kDa. Mg2+ ion is required for the enzyme action and the optimum pH for the reaction is pH 8.0. The enzyme hydrolyzes 8-oxodGTP to 8-oxodGMP with a Km value of 12.5 microM. dGTP and dATP are also degraded to dGMP and dAMP, respectively, with Km values 70 times greater than that for 8-oxodGTP. dTTP and dCTP are not hydrolyzed. These properties of the human 8-oxodGTPase are similar to those observed with the E. coli MutT protein, suggesting that the function of protecting the genetic information from the threat of endogenous oxygen radicals is widely distributed in organisms.