Liu Xiaoping, Samouilov Alexandre, Lancaster Jack R, Zweier Jay L
Molecular and Cellular Biophysics Laboratories, Department of Medicine, Division of Cardiology and the Electron Paramagnetic Resonance Center, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21224, USA.
J Biol Chem. 2002 Jul 19;277(29):26194-9. doi: 10.1074/jbc.M201939200. Epub 2002 May 10.
The process of NO transfer into erythrocytes (RBCs) is of critical biological importance because it regulates the bioavailability and diffusional distance of endothelial-derived NO. It has been reported that the rate of NO reaction with oxyhemoglobin (Hb) within RBCs is nearly three orders of magnitude slower than that by equal amounts of free oxyhemoglobin. Consistent with early studies on oxygen uptake by RBCs, the process of extracellular diffusion was reported to explain this much lower NO uptake by RBC encapsulated Hb (Liu, X., Miller, M. J., Joshi, M. S., Sadowska-Krowicka, H., Clark, D. A., and Lancaster, J. R., Jr. (1998) J. Biol. Chem. 273, 18709-18713). However, it was subsequently proposed that the RBC membrane provides the main resistance to NO uptake rather than the process of extracellular diffusion (Vaughn, M. W., Huang, K. T., Kuo, L., and Liao, J. C. (2000) J. Biol. Chem. 275, 2342-2348). This conclusion was based on competition experiments that were assumed to be able to determine the rate constant of NO uptake by RBCs without extracellular diffusion limitation. To test the validity of this hypothesis, we theoretically analyzed competition experiments. Here, we show that competition experiments do not eliminate the extracellular diffusion limitation. Simulation of the competition data indicates that the main resistance to NO uptake by RBCs is caused by extracellular diffusion in the unstirred layer surrounding each RBC but not by the RBC membrane. This extracellular diffusion resistance is responsible for preventing interference of NO signaling in the endothelium without the need for special NO uptake by intracellular hemoglobin or a unique membrane resistance mechanism.
一氧化氮（NO）转运至红细胞（RBCs）的过程具有至关重要的生物学意义，因为它调节内皮源性NO的生物利用度和扩散距离。据报道，RBCs内NO与氧合血红蛋白（Hb）的反应速率比等量游离氧合血红蛋白的反应速率慢近三个数量级。与早期关于RBCs摄取氧气的研究一致，据报道细胞外扩散过程可解释RBC包裹的Hb对NO摄取量低得多的现象（Liu, X., Miller, M. J., Joshi, M. S., Sadowska-Krowicka, H., Clark, D. A., and Lancaster, J. R., Jr. (1998) J. Biol. Chem. 273, 18709 - 18713）。然而，随后有人提出，RBC膜对NO摄取起主要阻力作用，而非细胞外扩散过程（Vaughn, M. W., Huang, K. T., Kuo, L., and Liao, J. C. (2000) J. Biol. Chem. 275, 2342 - 2348）。这一结论基于竞争实验，这些实验被认为能够在无细胞外扩散限制的情况下确定RBCs摄取NO的速率常数。为了检验这一假设的有效性，我们对竞争实验进行了理论分析。在此，我们表明竞争实验并未消除细胞外扩散限制。对竞争数据的模拟表明，RBCs摄取NO的主要阻力是由每个RBC周围未搅拌层中的细胞外扩散引起的，而非RBC膜。这种细胞外扩散阻力负责防止NO信号在内皮中受到干扰，而无需细胞内血红蛋白进行特殊的NO摄取或独特的膜阻力机制。
