All in the blood

HAEMOGLOBIN, the ubiquitous component of red blood cells (RBC), known to be a transporter of gases - ferrying oxygen from the lungs to tissues and carbon dioxide (c02) on the return journey - has been found to distribute a third gas on its rounds through the body. This gas is nitric oxide (NO), according to a team of researchers from Duke University Medical School, us, led by Jonathan Stamler.

NO, though known as a noxious gas in nature, is important in keeping cells and tissues alive. The cells which line blood vessels release NO in adjoining tissues. The gas has a relaxing effect on muscle cells and thus can increase blood flow. NO, hence, plays a role in altered blood flow at specific sites and also in related biological functions like memory formation, blood pressure and sexual erection.

Each haernoglobin sub-unit cradles an atom of iron which has a strong affinity for oxygen. The researchers isolated another component of haemoglobin, a segment of its protein chain called the cysteine residue, that can hold and release No at cellular sites. Haemoglobin thus directly controls the ability to regulate local levels Of NO in the circulatory system according to need. "People thought that they knew everything there was to know about haemoglobin. That it has another basic function is a stunning revelation," said Stamler.

A question Stamler's team members asked themselves was how muscle cells got their necessary fix of,nitric acid. When No acquires an extra electron, the Duke researchers observed, it takes on an altogether different chemistry. The more energetic NO molecules, dubbed 'super' No by the scientists, bind to the tail of the amino acid called cysteine, present in the beta units of a haemoglobin molecule. Stamler and his colleagues Li Jia and Joseph Bonaventura found that super NO is indeed present in the blood cells as they leave the lungs, but not in those returning to the lungs from the tissues. This implies that super NO is formed in the lungs and transported to the tissues. The team is now trying hard to find out the details of the mechanism.

This new discovery calls for a major revision in all textbook pictures of the respiratory cycle. When an RBc enters the lungs, its haemoglobin molecules release C02 and pick up oxygen and super NO. The RBC travel through the arteries and the, tiny capillaries where oxygen is released. Free of oxygen, the iron atoms trap any local excess Of NO making the blood vessels contract. Wherever necessary, super NO is released and the blood vessels expand. The RBc returning to the lungs dump the co2 and NO picked up from the tissues and get recharged with oxygen and super No; the cycle continues in a like manner.

Though the experiments at the Duke Medical School were performed on rats, Stamler believes that the same results would be found in human tissues because of the basic physiological similarities between rats and humans. The new finding suggests solutions to several medical mysteries. When the heart is deprived of oxygen - the most common cause of a heart attack - it also lacks in super NO. A @dose Of NO-rich haernoglobin may help restore the natural balance and ward off initial attacks.

The most important fallout of the new research, opines Nobel prize-winner Max Perutz of the molecular biology department of Cambridge University, UK (who discovered the crystal structure of haemoglobin) would be new ways to control blood pressure related diseases. A time may come when haemoglobin doped with the right amount Of NO can work like the proverbial magic potion to treat high blood pressure cases.

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