Cardiovascular Diseases: New Hemoglobin Function Discovered

In a paper published in Nature Chemical Biology, senior authors Daniel Kim-Shapiro, professor of physics at Wake Forest, and Mark Gladwin, chief of the Vascular Medicine Branch of the National Heart, Lung and Blood Institute of the NIH, describe how hemoglobin, through a catalytic reaction that does not change its own chemical properties, converts nitrite salt to the vasodilator nitric oxide. The paper further documents how the nitric oxide activity harnessed by hemoglobin escapes the red blood cell to regulate blood flow and how the process, surprisingly, relies on the oxidized, or rusted, form of hemoglobin, previously associated only with diseased states.

"We believe we have solved the paradox of how hemoglobin mediates the conversion of nitrite to nitric oxide in a way that it is not immediately destroyed in the red cell and so it can be effective biologically," Kim-Shapiro says.

In the bloodstream, iron-rich hemoglobin consumes, on contact, any free nitric oxide released by the blood vessels, so the idea that hemoglobin participates in forming nitric oxide had seemed implausible until recently.

In 2003, Gladwin and collaborators at the NIH, Wake Forest and the University of Alabama discovered that nitrite salt, the same substance used to cure meat and previously thought to be biologically inert, serves in the cell as a storage pool for nitric oxide. Since then, nitrite has been the object of intense study by researchers worldwide in pursuit of new treatments for such conditions as sickle cell disease, myocardial infarction, pulmonary hypertension, stroke and atherosclerosis.

In the most recent study, the researchers conclude that the nitrite-hemoglobin reaction generates dinitrogen trioxide (N₂O₃), which takes one of several pathways from the red blood cell and later separates into nitric oxide (NO) and nitrogen dioxide (NO₂).

The newly discovered chemistry in hemoglobin has eluded scientists for a century because the intermediate molecule, nitrite-methemoglobin, formed during the process cannot be observed by electron paramagnetic resonance spectroscopy, the most sophisticated analysis technique currently available. That has rendered the reaction "invisible" by direct observation, but indirect measurement of the process is possible.

"Using a variety of biophysical techniques and by careful examination of the rates of reactions and the products that are made when experimenting with hemoglobin and nitrite, we were able to discover this reaction mechanism," Kim-Shapiro explains.

The research was funded by grants from the NIH. (Research paper published November 4, 2007: dol10.1038/nchembio.2007.46)

Also contributing to the study were Swati Basu, research assistant professor of physics at Wake Forest; Rozalina Grubina, a Howard Hughes Scholar in Gladwin's NIH laboratory; S. Bruce King, professor of chemistry at Wake Forest; Jinming Huang, research assistant professor of chemistry at Wake Forest; Xiaojun He, postdoctoral fellow in physics at Wake Forest; Wake Forest graduate students Ivan Azarov and Anne Jeffers; Wake Forest undergraduate students Atul Mehta and Ryan Seibert; Wake Forest visiting undergraduate student Alice Jiang; Zhi Huang, a postdoctoral fellow in Gladwin's NIH laboratory, Jeanet Conradie, senior lecturer at the University of the Free State in South Africa; Rakesh Patel, associate professor of pathology at the University of Alabama at Birmingham; Neil Hogg, associate professor of biophysics at the Medical College of Wisconsin in Milwaukee; and Abhik Ghosh, professor of chemistry at the University of Tromsø in Norway.