Acceleration of the autoxidation of nitric oxide by proteins

Lipoproteins and lipid membranes accelerate ^•NO autoxidation by increasing local concentration of ^•NO and O₂. Although the idea that proteins could also accelerate this reaction was presented some time ago, it was largely criticized and dismissed. Herein the effect of proteins on ^•NO autoxidation rates was studied following ^•NO disappearance with a selective electrode. It was found that human serum albumin (HSA) accelerated ^•NO autoxidation by a factor of 9 per g/mL of protein, much less than previously suggested. The acceleration by HSA was sensitive to pH and significantly decreased at pH lower than 4.5 coincident with the acid structure transition of HSA to a partially unfolded and rigid conformation. Other proteins with different surface hydrophobicity also accelerated ^•NO autoxidation and it was found to depend mostly on the protein size and dynamics. Mathematical simulations were performed to assess the physiological importance of this acceleration. It was calculated that in plasma the autoxidation of ^•NO is accelerated 1.38 times by HSA relative to water alone, but this becomes of little relevance when whole blood is simulated because of the rapid rate of ^•NO consumption by red blood cells.