Comment on Dejam et al, page 734
In 1992, the journal Science named nitric oxide (NO) the “molecule of the year.”1 In the 13 years that have elapsed since this honorific was bestowed, the biomedical community has become “fixated” on the biologic role of NO and its metabolites. Scientific advances have, at a logarithmic pace, affected virtually every aspect of systems physiology in health and disease.
It has long been appreciated that the blood contains small levels of the NO oxidation product nitrate and even lower levels of nitrite. In addition to a variable on-going supply of these products from the diet, nitrate and, to a lesser extent, nitrite can be readily formed from the reaction of endogenous NO with oxyhemoglobin. Until recently, these anions have been considered biologic end products with little or no physiologic significance. A group at the National Institutes of Health (NIH) led by Mark Gladwin and Alan Schechter has accumulated evidence that nitrite in the plasma can be converted to NO by means of intracellular hemoglobin serving as a nitrite reductase.2 This source of NO could make an important contribution to the regulation of vasomotor tone. Accurate measurements of blood nitrite are required to properly test this hypothesis.
In the paper by Dejam and colleagues in this issue, rigorous analytic methods have been used to measure nitrite levels in the plasma and red cells. The authors report a small but significant drop in nitrite levels as blood circulates from arteries to veins, a finding consistent with conversion of nitrite to NO, and perhaps other products, during flow through the microcirculation. Other recent studies from the NIH group suggest that hemoglobin's catalytic role as a nitrite reductase is maximal with the degree of partial deoxygenation encountered during capillary blood flow.3 In order for this enzymatic function of hemoglobin to be efficient, there must be adequate transit of nitrite into red cells. Indeed, Dejam et al have found nitrite levels in the erythrocyte cytosol that are 2-fold those in the plasma. It is expected that the band 3 anion channel would determine the partition of nitrite between the plasma and the erythrocyte cytosol. If so, this distribution should follow the Gibbs-Donnan equilibrium. Because hemoglobin and 2,3-bisphosphoglycerate (2,3-BPG) are abundant impermeant anions inside the red cell, the concentration of cytosolic nitrite should be about 0.7 that in the plasma, similar to the ratios for chloride or bicarbonate. Thus, the very high nitrite levels in the red cell are quite unexpected and suggest binding somewhere within the cytosol. This apparent conundrum merits further inquiry.
In pursuing the biologic significance of blood nitrite levels, it is anticipated that further studies will provide additional pertinent information. Is the level of plasma nitrite affected by diet, renal function, and/or liver function? Although endogenous NO production from endothelial nitric oxide synthase (NOS) may be the major source of plasma nitrite, other sources should be explored. Macrophage NOS (iNOS) can be induced 2 to 3 orders of magnitude following inflammation. Are blood nitrite levels influenced by infection, cancer, or connective tissue disorders? Such information is highly germane to the issue of whether blood nitrite serves as an important source of NO for regulation of vasomotor tone.
The evolution of civilization has relied, in part, on the fixation of atmospheric nitrogen into nitrates and nitrites, enabling the development of a range of products including fertilizers and explosives. The process in which nitrite is “defixated,” converted into biologically active NO, may also turn out to have important and widespread biologic significance. ▪
