在真核细胞中,许多细胞外刺激是通过第二信使而起作用的,研究最多的第二信使是cAMP和cGMP,而且一般认为,这些环核苷酸是通过各自的依赖性蛋白激酶起作用的,这些激酶对细胞内二种环核苷酸水平的改变可发生反应,从而改变关键蛋白的磷酸化状态。近来一些人对这种观点提出了异议,他们认为:在许多情况下,环核苷酸的转换是很重要的,而其浓度是不重要的。Goldberg等建 In eukaryotic cells, many extracellular stimuli work through the second messenger. The most studied second messengers are cAMP and cGMP, and it is generally believed that these cyclic nucleotides act through their respective dependent protein kinases. These kinases respond to changes in the levels of two cyclic nucleotides in the cell, altering the phosphorylation status of key proteins. Recently, some people have disputed this view. They believe that in many cases, the cyclic nucleotide conversion is very important, and its concentration is not important. Goldberg et al.