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J Appl Physiol (July 5, 2007). doi:10.1152/japplphysiol.00443.2007
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Submitted on April 24, 2007
Accepted on July 3, 2007

A novel method of measuring reduction of nitrite-induced methemoglobin applied to fetal and adult blood of humans and sheep

Gordon G. Power1*, Shannon L. Bragg1, Bryan T. Oshiro2, Andre Dejam3, Christian J. Hunter3, and Arlin B. Blood4

1 Perinatal Biology, Loma Linda University, Loma Linda, California, United States
2 Department of Obstetrics and Gynecology, Division of Perinatal Medicine, Loma Linda University School of Medicine, Loma Linda, California, United States
3 Department of Medicine, Brigham and Women's Hospital, Boston, Massachusetts, United States
4 Department of Pediatrics, Division of Neonatology, Loma Linda School of Medicine, Loma Linda, California, United States

* To whom correspondence should be addressed. E-mail: gpower{at}llu.edu.

The reaction of nitrite with deoxyhemoglobin results in the production of nitric oxide and methemoglobin, a reaction recently proposed as an important oxygen-sensitive source of vasoactive nitric oxide during hypoxic/anoxic stress, with several animal studies suggesting nitrite may have therapeutic potential. Accumulation of toxic levels of methemoglobin is suppressed by reductase enzymes present within the erythrocyte. Using a novel method of measuring methemoglobin reductase activity in intact erythrocytes, we compared fetal and adult sheep and human blood. Following nitrite-induced production of 20% methemoglobin the blood was equilibrated with carbon monoxide, which effectively stopped further production. Methemoglobin disappearance was first-order in nature with specific rate constants, k x 1000, of 12.9±1.3 min-1 for fetal sheep, 5.88±0.26 min-1 for adult sheep, 4.27±0.34 for adult humans, and 3.30±0.15 for newborn cord blood, all statistically different from one another. The effects of oxygen tensions, pH, hemolysis, and methylene blue are reported. Studies of temperature dependence indicated an activation energy of 8620±1060 cal mol-1 (2.06 kJ mol-1), appreciably higher than would be characteristic of processes limited by passive membrane diffusion. In conclusion, the novel methodology permits absolute quantification of the reduction of nitrite-induced methemoglobin in whole blood.




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