Our red blood cells are packed with hemoglobin, which ferries oxygen from the lungs out to the body, where it picks up carbon dioxide and carries it back to the lungs to be exhaled. It’s a marvelous, well-known cycle. Yet last March researchers at Duke University Medical Center reported the discovery of something fundamentally new about the role of hemoglobin--a second chemical cycle that aids oxygen delivery.
The cycle involves nitric oxide (NO), which blood vessels secrete to dilate themselves. Like oxygen, NO can bind to the iron atom in hemoglobin--which had led some researchers to wonder why hemoglobin doesn’t sweep all the NO out of the bloodstream, constrict blood vessels, and send blood pressure through the roof. But cardiologist Jonathan Stamler and biochemist Joseph Bonaventura discovered that hemoglobin picks up NO not just in the blood but also in the lungs--where it’s produced in a chemically different form that can bind to the sulfur atom in hemoglobin. The resulting molecule, SNO, is carried into the bloodstream along with the iron-bound oxygen.
As a red blood cell travels to the tissue, Bonaventura explains, its hemoglobins begin releasing their oxygen cargo. That causes their structure to change, which triggers the release of SNO. And that paves the way for the red blood cell to open up a vessel so that it can barrel on through there and deliver more oxygen and pick up carbon dioxide.
At the same time, the hemoglobin is picking up NO. The red blood cell then transports both NO and CO2 back to the lungs to be exhaled. The finding that nitric oxide cycles in the body was a big surprise, says Bonaventura. It makes the red blood cell a fundamental part in the control of blood pressure.