Mo J Y, Maki H, Sekiguchi M
Department of Biochemistry, Faculty of Medicine, Kyushu University, Fukuoka, Japan.
Proc Natl Acad Sci U S A. 1992 Nov 15;89(22):11021-5. doi: 10.1073/pnas.89.22.11021.
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.
8-氧代鸟嘌呤核苷酸能以几乎相同的效率与胞嘧啶和腺嘌呤核苷酸配对。一旦在细胞核苷酸池中形成8-氧代鸟苷三磷酸（8-oxodGTP），这种致突变核苷酸就会掺入DNA中并导致颠换突变。大肠杆菌的MutT蛋白具有将8-oxodGTP水解为相应单磷酸核苷的酶活性，因此可能负责防止此类突变的发生。在此我们表明，人类细胞有一种酶，能以类似于MutT蛋白的方式特异性水解8-oxodGTP。已在Jurkat细胞的无细胞提取物中发现人8-氧代鸟苷三磷酸酶（8-oxodGTPase），并将其纯化了400多倍。通过凝胶过滤和凝胶电泳分析表明，人8-oxodGTPase天然形式的分子量为18 kDa。酶促反应需要Mg2+离子，反应的最适pH为8.0。该酶将8-oxodGTP水解为8-氧代鸟苷单磷酸（8-oxodGMP），Km值为12.5 microM。dGTP和dATP也分别降解为dGMP和dAMP，其Km值比8-oxodGTP的Km值大70倍。dTTP和dCTP不被水解。人8-oxodGTPase的这些特性与在大肠杆菌MutT蛋白中观察到的特性相似，这表明保护遗传信息免受内源性氧自由基威胁的功能在生物中广泛存在。
