Hickie R A, Walker C M, Datta A
Cancer Res. 1975 Mar;35(3):601-5.
The total cyclic adenosine 3':5'-monophosphate (cAMP) phosphodiesterase activities as well as the activities of the low- and high-K-m enzyme forms were investigated in homogenates, 100,000 X g supernatants, and plasma membrane fractions of rat liver and Morris hepatoma 5123tc(h); the responsiveness of hepatoma and liver plasma membrane (low-K-m) phosphodiesterases to imidazole (40 mM) and theophylline (5mM) were also compared at cAMP concentrations of 1 and 7.5 muM. The total cAMP phosphodiesterase activities of tumor homogenates and 100,000 X g supernatant fractions were found to be less than one-half those of liver; kinetic studies of homogenates indicated that this finding was largely due to a substantial reduction (53%) in activity of the hepatoma high-K-m enzyme. In contrast, low-Km cAMP phosphodiesterase activities for tumor homogenate and plasma membrane fractions were significantly (50%) higher than liver; this was particularly evident when cAMP concentrations were between 0.5 and 2 muM. Since these concentrations are in the range of basal physiological levels of cAMP in hepatocytes, the present results suggest that the reduced levels of cAMP, previously observed in hepatoma 5123tc (h), are primarily due TO An increased rate of cAMP metabolism by low-Km cAMP phosphodiesterase in plasma membranes of the tumor. Imidazole increased the activity of the low-K-m cAMP phosphodiesterase of liver plasma membranes by 22 (1 muM cAMP) and 38% (7.5 muM camp); tumor activity was enhanced 35 and 50%, respectively, at 1 and 7.5 muM cAMP. Theophylline inhibited the plasma membrane phosphodiesterase activity of liver 79 and 53% at cAMP concentrations of 1 and 7.5 muM, respectively; hepatoma activity was inhibited 82 (1 muM cAMP) and 62% (7.5 muM cAMP).
研究了大鼠肝脏和莫里斯肝癌5123tc(h)的匀浆、100,000×g上清液及质膜组分中总环磷酸腺苷（cAMP）磷酸二酯酶活性以及低Km和高Km酶形式的活性；还比较了肝癌和肝质膜（低Km）磷酸二酯酶在1和7.5μM cAMP浓度下对咪唑（40 mM）和茶碱（5 mM）的反应性。发现肿瘤匀浆和100,000×g上清液组分的总cAMP磷酸二酯酶活性不到肝脏的一半；匀浆的动力学研究表明，这一发现主要是由于肝癌高Km酶活性大幅降低（53%）。相反，肿瘤匀浆和质膜组分的低Km cAMP磷酸二酯酶活性比肝脏显著高（50%）；当cAMP浓度在0.5至2μM之间时，这种情况尤为明显。由于这些浓度处于肝细胞中cAMP的基础生理水平范围内，目前的结果表明，先前在肝癌5123tc(h)中观察到的cAMP水平降低主要是由于肿瘤质膜中低Km cAMP磷酸二酯酶使cAMP代谢速率增加。咪唑使肝质膜低Km cAMP磷酸二酯酶活性在1μM cAMP时增加22%，在7.5μM cAMP时增加38%；在1和7.5μM cAMP时，肿瘤活性分别增强35%和50%。茶碱在cAMP浓度为1和7.5μM时分别抑制肝脏质膜磷酸二酯酶活性79%和53%；肝癌活性在1μM cAMP时被抑制82%，在7.5μM cAMP时被抑制62%。
