Reply to Sakai: What Is the Major Mechanism of Slower NO Uptake by Red Blood Cells

This is a response to a letter by Hiromi Sakai (1)

In their original work Sakai and colleagues (2) suggested that internal diffusion alone can account for slower NO uptake by hemoglobin-containing vesicles. Our simulations are consistent with this conclusion, but we show that better agreement between experiment and theory is obtained when all three mechanisms (internal diffusion, external diffusion, and membrane permeability) are included. Moreover, our simulations show that for phospholipid vesicles and red blood cells, external diffusion has the largest effect. Importantly, our experimental data showing that viscosity of the external medium affects NO uptake by red cells provide unequivocal evidence for a role of external diffusion, contrary to the position held by Sakai. This same approach, combining experimental and theoretical data, showed that external diffusion is the main factor limiting oxygen uptake by red cells 30 years ago, and one would not expect a big difference with NO (3, 4).

We have always held that membrane permeability plays a small role in limiting NO uptake compared to other factors in most cases. However, in the case of red cell microparticles, we find that it plays a major role. It should be remembered that these microparticles only take up NO a few times slower than free hemoglobin (whereas red cells take up NO up to 1000 times slower), so the effect is still small. Like Professor Liao (5, 6), we suggest the effects on membrane permeability are due to the protein scaffold, and this notion is strongly supported by data in which the protein scaffold was physically or chemically modified (6).