Question: 

Is myoglobin (Mb)- and haemoglobin(Hb)-facilitated O₂ diffusion limited by the rate of the deoygenation reaction?

Methodology: 

A mathematical model describing facilitation of O₂ diffusion by diffusion of Mb and Hb and their simultaneous reaction with O₂ is presented. The equations are solved numerically by a finite-difference method with the known kinetic constants of the oxygenation-deoxygenation reactions and for the conditions prevailing in cardiac and skeletal muscle and in red cells, respectively.

Result: 

It is demonstrated that for the relevant intracellular diffusion distances, the degree of facilitation is limited by the rate of chemical reaction between Mb, Hb and O₂. It is calculated that the limitation by reaction kinetics reduces the maximally possible facilitated O₂ diffusion in cardiomyoctes by ~50%, and in skeletal muscle fibers by ~20%. For human red blood cells, a reduction of maximal facilitated O₂ diffusion by 50% is obtained, in agreement with previous reports.

Conclusion: 

In cardiomyocytes and red cells, chemical equilibrium between Mb or Hb and O₂, quite commonly assumed in many studies of muscular O₂ transport, is far from being established. This implies that in cardiac muscle cells, and to a lesser extent also in skeletal muscle, the "O₂ transport function of Mb" is severely limited by chemical reaction kinetics. However, the "storage function of Mb", e.g. the speed of release of O₂ from MbO₂ during systolic reduction of coronary perfusion and the associated rapid fall in intracellular pO₂, is not limited by reaction kinetics. We conclude that the O₂ storage function of Mb is of greater significance than the O₂ transport function.