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"We always knew that the hemoglobin complex had two reactive arms, a heme and a cysteine, to which other molecules could attach, but no one knew what the cysteine's function was," he said. "We now know that it serves to bind NO."

Further experimentation by the group uncovered the intricate interplay between hemoglobin, oxygen, NO and carbon dioxide:

• After hemoglobin loads up oxygen and SNO in the lung, the hemoglobin-trio travels down arteries through the heart and into the rest of the body to deliver its load of oxygen. That process has to happen with open vessels and in a constant pressure. So as the hemoglobin complex inevitably devours the free NO gas that endothelial cells produced to dilate the vessel, hemoglobin simultaneously releases the SNO molecule it had been carrying. One "ineffective" NO is exchanged for another "protected" NO. Blood pressure remains constant and blood flow maintained in order to promote oxygen transport. Hemoglobin detects how much NO has been removed from the bloodstream and compensates with SNO, the researchers believe. Still, the hemoglobin-SNO molecules are very abundant, so there is still plenty of SNO still available as the hemoglobin complex enters tissues.
  
  
• In tissue, the hemoglobin undergoes a major structural change to release its load of oxygen. This same "conformational" change also releases SNO, presumably to increase the efficiency of oxygen utilization, Stamler said. "Mitochondria in tissue make energy from oxygen, and the SNO may help regulate the rate at which the mitochondria respire, or use the oxygen." It is also possible that SNO regulates capillary blood flow, he said.
  
  
• When the hemoglobin has released both its oxygen and SNO, it can attract molecules of carbon dioxide. Carbon dioxide (CO2) is the waste gas produced from oxygen respiration. Hemoglobin binds CO2 and carries it to the lungs, where it is exhaled. At that point, t
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Contact: Renee Twombly
twomb001@mc.duke.edu
919-660-1313
Duke University
29-Apr-1996


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