Liau M C, Hunt M E, Hurlbert R B
Biochemistry. 1976 Jul 13;15(14):3158-64. doi: 10.1021/bi00659a033.
The activity of rRNA methylases was stimulated by high-energy precursors of RNA (ribonucleoside triphosphates) and inhibited by degradation products of RNA (ribonucleotides and oligoribonucleotides). The response of methylases from rat Novikoff ascites tumor and liver to these metabolites was strikingly different. The highly active tumor enzymes responded preferentially to inhibition by catabolic metabolites, whereas the less active liver enzymes responded exclusively to stimulation by anabolic metabolites. When the activity of rRNA methylases was assayed in response to increasing concentration of S-adenosylmethionine, the tumor enzymes responded with a hyperbolic substrate dependence curve and the liver enzymes with a sigmoidal curve. In the presence of an inhibitory dinucleotide, ApA, the tumor enzymes responded with a sigmoidal curve; in the presence of a stimulator, adenosine 5'-triphosphate, the liver enzymes responded with a hyperbolic substrate concentration curve. When normal rats were subject to a series of treatments by thioacetamide, a hepatocarcinogen, the liver nucleolar rRNA methylases became responsive to inhibition by ApA and relatively unresponsive to stimulation by adenosine 5'-triphosphate. When tumor-bearing rats were treated with polyinosinate:polycytidylate, an antitumor agent, the tumor nucleolar rRNA methylases became unresponsive to inhibition by ApA and more responsive to stimulation by adenosine 5'-triphosphate. A correlation was noted between increased methylation efficiency in vivo and increased stability of nucleolar RNA during incubation in vitro, or vice versa. These results are interpreted to indicate that rRNAmethylases are regulated by cellular metabolites during the nucleolar biosynthesis of ribosomes and that rRNA methylases may provide a favorable site for selective action by cancer chemotherapeutic agents.
rRNA甲基化酶的活性受到RNA的高能前体（核糖核苷三磷酸）的刺激，并受到RNA降解产物（核糖核苷酸和寡核糖核苷酸）的抑制。来自大鼠诺维科夫腹水瘤和肝脏的甲基化酶对这些代谢物的反应显著不同。高活性的肿瘤酶优先对分解代谢物的抑制作出反应，而活性较低的肝脏酶仅对合成代谢物的刺激作出反应。当测定rRNA甲基化酶对S-腺苷甲硫氨酸浓度增加的反应时，肿瘤酶呈现双曲线型底物依赖性曲线，而肝脏酶呈现S型曲线。在存在抑制性二核苷酸ApA的情况下，肿瘤酶呈现S型曲线；在存在刺激物三磷酸腺苷的情况下，肝脏酶呈现双曲线型底物浓度曲线。当正常大鼠接受硫代乙酰胺（一种肝癌致癌物）的一系列处理时，肝脏核仁rRNA甲基化酶变得对ApA的抑制有反应，而对三磷酸腺苷的刺激相对无反应。当荷瘤大鼠用聚肌苷酸：聚胞苷酸（一种抗肿瘤剂）处理时，肿瘤核仁rRNA甲基化酶变得对ApA的抑制无反应，而对三磷酸腺苷的刺激更有反应。在体内甲基化效率增加与体外孵育期间核仁RNA稳定性增加之间发现了相关性，反之亦然。这些结果被解释为表明rRNA甲基化酶在核糖体的核仁生物合成过程中受细胞代谢物调节，并且rRNA甲基化酶可能为癌症化疗药物的选择性作用提供一个有利位点。
