Hemoglobin: Nitrite Reductase or Nitric Oxide Scavenger?

Abstract

Most recent studies have reported that deoxyghemoglobin can reduce nitrite leading to nitric oxide (NO) generation to elicit vasodilation. However, questions remain concerning how NO escapes the trapping of abundant hemoglobin in red blood cells. Thus, controversy remains regarding whether hemoglobin functions an important nitrite reductase involved in hypoxic vasodilation or as a NO scavenger blocking NO-mediated vasodilation. To characterize the enzymatic role of hemoglobin, spectrophotometric measurements, Electron Paramagnetic Resonance (EPR) spectroscopy, chemiluminescence NO analyzer, and NO electrode studies were performed. Both EPR spectroscopy and spectophotometric studies showed formation of methemoglobin and nitrosylhemoglobin upon incubation of deoxyhemoglobin with nitrite. In the presence of excess of hemoglobin, no NO generation was detected. Thus, it is certain that hemoglobin can react with nitrite, however, NO generated was trapped by ferro-hemoglobin. Using 5 ml blood instead of hemoglobin, addition of 0.1 mM nitrite triggered NO generation with a rate of 20 pM/s. In blood pretreated with CO, addition of 0.1 mM nitrite triggered NO generation with a rate four times higher than that from untreated blood. Thus, hemoglobin functioned as an NO scavenger. We further investigated nitrite reduction in the presence of 0.2 g/ml liver, heart, or aortic ring tissue pieces in PBS solution. The NO generation rates from 0.1 mM nitrite were ~ 4.2 nM/s (liver), 3.1 nM/s (heart), or 1.7 nM/s (aortic ring), respectively. Overall, these results indicate that nitrite reduction to NO is widely present in different tissues and blood; however, hemoglobin when present in excess appears to function as a NO scavenger rather than as a source of free NO.