What Is the Major Mechanism of Slower NO Uptake by Red Blood Cells?

Azarov et al. (1), using our experimental data (2), resimulated the mechanism of slower NO uptake by RBCs and hemoglobin (Hb)-encapsulated vesicles. We inferred the contribution of the intracellular diffusion barrier and the absence of the membrane barrier. Although acknowledging an intracellular diffusion barrier, they insisted on the presence of a membrane barrier and a major contribution of an extracellular diffusion barrier. That presents some problems.

First, the membrane permeability of NO they used, 9 × 10⁵ μm s⁻¹, originated from Subczynski et al. (3), who clarified that the permeability is equivalent to or greater than that of a water layer of equal thickness. This supports the absence of a membrane barrier.

Second, the diffusion constant of NO they used, 3300 μm² s⁻¹ (37 °C) (4), is much higher than that in other papers (2, 3) and our intracellular diffusion constants (25 °C), which they used for simulation (2). The positional relation between a nanoparticle and an extracellular phase is fixed, in spite of the rapid motion of nanoparticles by the stopped-flow method. Accordingly, they might overestimate the extracellular barrier.

Third, they suggest a cytoskeleton barrier. However, it is difficult to imagine the existence of more NO-binding and -reacting sites in a thin cytoskeletal layer than the abundant sites of the adjacent intracellular Hb solution. To measure the NO-binding rate constant, they used the gentle “competition method,” which presents the possibility of slow motion and sedimentation of RBCs. Therefore, the extracellular and membrane barriers might be overestimated.

Their simulation requires corrections to improve its reliability.