NEW FUNCTIONS FOR AN OLD MOLECULE; HEMOGLOBIN AS REGULATOR OF BLOOD PRESSURE Hemoglobin is one of the widely studied groups of proteins. Now, the team of Jonathan Stamler in the last issue of Nature in March 1996 report a new function for this protein. These scientists discovered that hemoglobin interacts with nitric oxide (NO) to regulate the blood pressure. NO is made by the endothelial cells and by virtue of its action on the vascular smooth muscle leads to vasodilation. The interaction of NO with hemoglobin leads to the formation of S-nitrosothiols or SNO with an enhanced capability. SNO is formed when oxidized NO interacts with the highly reactive thiol groups on the two cystein residues in the hemoglobin molecule. Whereas native hemoglobin led to the elevation of blood pressure, the SNO-hemoglobin did not exert the same effect. The amount of such hemoglobin was found to be high in the oxygenated blood in the left side of the heart and to be low in the poorly oxygenated blood in the right side of the heart. Therefore, it seems that the allosteric changes that lead to the release of CO2 from the hemoglobin and its oxygenation are also associated with formation of SNO. The reverse occurs in the tissues. Simultaneous with the release of oxygen in the tissues, SNO is also released from the hemoglobin. Therefore, the function of hemoglobin does not seem to be merely confined to its vital role of tissue oxygenation, rather this molecule actively participates in the regulation of blood pressure.