Backes W L, Means M, Canady W J
J Biol Chem. 1984 Aug 25;259(16):10092-9.
The substrates hexobarbital and ethylbenzene have been shown to compete for the spectral binding site of phenobarbital-induced rat hepatic microsomal cytochrome p-450. The two substrates produce different delta Absmax values, and the presence of one substrate does not affect the delta Absmax of the other substrate and vice versa. The respective binding constants for the two substrates are similarly unaffected. The conclusion drawn from these observations is that, over the concentration ranges studied, there is no change in the availability of the enzyme as a result of substrate addition; the difference in delta Absmax apparently being due to varying abilities of different substrates to bring about a spin shift in the enzyme. Evidence is presented to indicate that differences between enzymes from untreated male rats and phenobarbital-treated male rats are attributable to differences in the enzyme itself and not to changes in the nature of the membrane brought about by phenobarbital administration, at least insofar as heat entropy compensation is concerned. The enthalpy-entropy compensation observed in the binding of a homologous series of barbiturates to the microsomal membrane as determined from the membrane concentration dependence of their binding constants is shown to agree surprisingly well with the direct determination performed by Sitar and Mannering.
已表明底物己巴比妥和乙苯会竞争苯巴比妥诱导的大鼠肝微粒体细胞色素P - 450的光谱结合位点。这两种底物产生不同的最大吸光度变化值（delta Absmax），并且一种底物的存在不会影响另一种底物的delta Absmax，反之亦然。两种底物各自的结合常数同样不受影响。从这些观察结果得出的结论是，在所研究的浓度范围内，添加底物不会导致酶的可利用性发生变化；delta Absmax的差异显然是由于不同底物使酶产生自旋位移的能力不同。有证据表明，未处理的雄性大鼠和经苯巴比妥处理的雄性大鼠的酶之间的差异可归因于酶本身的差异，而不是由于苯巴比妥给药导致膜性质的变化，至少就热熵补偿而言是这样。从一系列巴比妥类药物与微粒体膜结合的结合常数的膜浓度依赖性确定的焓 - 熵补偿，与西塔尔和曼纳林进行的直接测定结果惊人地一致。
