Walker M S, Becker F F
Cancer Biochem Biophys. 1981;5(3):169-73.
DNA from transplantable hepatocellular carcinoma (THC) 252 has recently been found to have a lower 5-methylcytosine content than DNA from normal or regenerating rat liver. We have determined that DNA methylase, purified 200-fold from nuclei of regenerating rat liver, can add more methyl groups to THC 252 DNA than to DNA from normal or regenerating rat liver. Furthermore, a similarly purified DNA methylase from THC 252 was found to methylate THC 252 DNA at a higher rate than it methylated DNA from normal or regenerating liver. The larger number of unmethylated sites in THC 252 DNA was not due to a deficiency of DNA methylase since the level of methylase activity of nuclear extracts from THC 252 was 2.7 times that of normal liver and 1.5 times that of regenerating liver. Methylases from these three sources had similar rats of reaction with different DNA substrates. These findings suggest that the hypomethylation of THC 252 DNA is not due to decreased methylase activity or to altered enzyme specificity.
最近发现，可移植性肝细胞癌（THC）252的DNA中5-甲基胞嘧啶含量低于正常或再生大鼠肝脏的DNA。我们已经确定，从再生大鼠肝脏细胞核中纯化200倍的DNA甲基化酶，与正常或再生大鼠肝脏的DNA相比，能给THC 252 DNA添加更多的甲基基团。此外，从THC 252中纯化得到的类似DNA甲基化酶，甲基化THC 252 DNA的速率高于其甲基化正常或再生肝脏DNA的速率。THC 252 DNA中未甲基化位点数量较多并非由于DNA甲基化酶缺乏，因为THC 252细胞核提取物的甲基化酶活性水平是正常肝脏的2.7倍，是再生肝脏的1.5倍。这三种来源的甲基化酶与不同DNA底物的反应速率相似。这些发现表明，THC 252 DNA的低甲基化并非由于甲基化酶活性降低或酶特异性改变。
