Gettys T W, Blackmore P F, Redmon J B, Beebe S J, Corbin J D
J Biol Chem. 1987 Jan 5;262(1):333-9.
A recent study showed that cAMP analogs lowered cAMP levels in rat hepatocytes (Corbin, J.D., Beebe, S.J., and Blackmore, P.F. (1985) J. Biol. Chem. 260, 8731-8735). The present work demonstrates that cAMP analogs also lowered cAMP in a rapid, concentration-dependent manner in heart and fat cells. In order to determine if the cAMP-dependent protein kinase mediated this effect, techniques were developed to assay the protein kinase activity ratio in hepatocytes treated with cAMP analogs. The activation of protein kinase and phosphorylase in hepatocytes by 8-pCl phi S-cAMP (where 8-pCl phi S- indicates 8-parachlorothiophenyl-) was concentration-dependent and occurred in parallel to proportionate decreases in cAMP. More than 20% of the cAMP binding sites on the protein kinase were unoccupied at concentrations of 8-pCl phi S-cAMP that produced maximal cAMP lowering. Thus, the possibility that 8-pCl phi S-cAMP lowered cAMP by displacing it from protein kinase binding sites, making it available for hydrolysis, seemed unlikely. In adipocytes, the lowering of cAMP by 8-pCl phi S-cAMP occurred in parallel with increases in lipolysis and activation of low Km phosphodiesterase, suggesting that the phosphodiesterase was responsible for the cAMP lowering. Further evidence for this assertion was the finding that in hepatocytes preloaded with low concentrations of 8-pCl phi S-cAMP, glucagon lowered 8-pCl phi S-cAMP by about 50%, an amount similar to the cAMP lowering observed with 8-pCl phi S-cAMP treatment. The results were consistent with a cAMP-dependent protein kinase-catalyzed activation of a phosphodiesterase and suggested that 8-pCl phi S-cAMP-mediated hydrolysis of cAMP mimicked a physiologically significant response. The observation of this phenomenon in several tissues further suggested that it may be a general mechanism for dampening and terminating the hormonal signal through accelerated degradation of cAMP.
最近的一项研究表明，环磷酸腺苷（cAMP）类似物可降低大鼠肝细胞中的cAMP水平（科尔宾，J.D.，毕比，S.J.，和布莱克莫尔，P.F.（1985年）《生物化学杂志》260卷，8731 - 8735页）。目前的研究表明，cAMP类似物还能以快速、浓度依赖性的方式降低心脏和脂肪细胞中的cAMP水平。为了确定cAMP依赖性蛋白激酶是否介导了这种效应，开发了一些技术来检测用cAMP类似物处理的肝细胞中的蛋白激酶活性比。8 - 对氯苯硫基-cAMP（其中8 - pCl phi S - 表示8 - 对氯苯硫基 - ）对肝细胞中蛋白激酶和磷酸化酶的激活呈浓度依赖性，并且与cAMP的相应比例降低同时发生。在产生最大cAMP降低效果的8 - 对氯苯硫基 - cAMP浓度下，蛋白激酶上超过20%的cAMP结合位点未被占据。因此，8 - 对氯苯硫基 - cAMP通过将cAMP从蛋白激酶结合位点上置换下来，使其可用于水解从而降低cAMP的可能性似乎不大。在脂肪细胞中，8 - 对氯苯硫基 - cAMP降低cAMP的同时伴随着脂解作用的增强和低Km磷酸二酯酶的激活，这表明磷酸二酯酶是导致cAMP降低的原因。这一论断的进一步证据是，在预先加载低浓度8 - 对氯苯硫基 - cAMP的肝细胞中，胰高血糖素使8 - 对氯苯硫基 - cAMP降低了约50%，这一降低量与用8 - 对氯苯硫基 - cAMP处理时观察到的cAMP降低量相似。这些结果与cAMP依赖性蛋白激酶催化激活磷酸二酯酶一致，并表明8 - 对氯苯硫基 - cAMP介导的cAMP水解模拟了一种生理上重要的反应。在多个组织中观察到这一现象进一步表明，它可能是通过加速cAMP降解来减弱和终止激素信号的一种普遍机制。
