Sleep J A, Hackney D D, Boyer P D
J Biol Chem. 1978 Aug 10;253(15):5235-8.
The change in the distribution of the phosphate species containing 0 to 4 18O oxygens per Pi was investigated during medium Pi equilibrium HOH exchange catalyzed by myosin subfragment 1. At 25 degrees C, a Pi molecule once bound loses an average of 3.9 of its original 4 oxygens prior to release which means that at least 100 reversals of the exchange reaction must have occurred. At 0 degrees C, only 3.4 of the 4 oxygens are lost prior to release indicating an average of 17 reversals. Distribution patterns are consistent with equivalent participation in the exchange reactions of all 4 oxygens of bound Pi. The intermediate exchange of Pi oxygens during hydrolysis of 18O-labeled ATP by myosin has also been investigated. The distribution of the product Pi species shows that there is an ATPase component in myosin preparations which hydrolyzes ATP without intermediate exchange. Presence of this component, which is likely a contaminating ATPase, provides a simple explanation of the apparent nonequivalence of phosphate oxygens which has been observed. When correction is made for this contaminant, characteristics of the myosin intermediate Pi equilibrium HOH exchange are similar to those of myosin subfragment 1 medium exchange, and intermediate exchange data are in much closer agreement with other kinetic measurements.
在肌球蛋白亚片段1催化的中等磷酸根（Pi）平衡水-水交换过程中，研究了每个Pi含有0至4个18O氧原子的含磷物种分布的变化。在25℃下，一个一旦结合的Pi分子在释放之前平均失去其原来4个氧原子中的3.9个，这意味着交换反应至少发生了100次逆转。在0℃下，释放之前4个氧原子中只有3.4个丢失，表明平均有17次逆转。分布模式与结合Pi的所有4个氧原子在交换反应中的同等参与是一致的。还研究了肌球蛋白对18O标记的ATP水解过程中Pi氧原子的中间交换。产物Pi物种的分布表明，肌球蛋白制剂中存在一种ATP酶成分，它在没有中间交换的情况下水解ATP。这种成分可能是一种污染性ATP酶，它的存在为观察到的磷酸根氧原子明显不等价现象提供了一个简单的解释。当对这种污染物进行校正后，肌球蛋白中间Pi平衡水-水交换的特征与肌球蛋白亚片段1中等交换的特征相似，并且中间交换数据与其他动力学测量结果更接近一致。
