216 VI. HEMOGLOBIN 



N \ N N 



glohin Fe: C^C): glohin Fe==C=0: 



x-\ /'-\ 



N N X N 



A B 



Carboxyhemoglobin, in common with other carbon monoxide-heme 

 compounds, is sensitive to light, the reaction affected being the disso- 

 ciation HbCO — ^ CO + Hb. Four light quanta are required for the 

 removal of one molecule of carbon monoxide from one iron atom of 

 carboxyhemoglobin but only one quantum is necessary with myocar- 

 boxyhemoglobin {37 Jf.). 



It is often essential to work with hemoglobin solutions in the 

 absence of oxygen and for many purposes the formation of carboxy- 

 hemoglobin is sufficient to cut down oxidative changes in the por- 

 phyrin structure to a minimum. Since no reagents are available 

 which readily combine irreversibly with carbon monoxide, without at 

 the same time injuring the protein, evacuation and repeated washing 

 with inert gases is the only means of removing the carbon monoxide 

 from carboxyhemoglobin, a process facilitated by strong illumination. 



2.2.4. Nitric Oxide Hemoglobin (HbNO). This compound was first observed 

 by Hermann in 18G5 {124-7) and may be formed in the absence of oxygen by 

 the reaction between nitric oxide and hemoglobin, but for manj' purposes it 

 is sufficient to reduce a mixture of oxyhemoglobin and sodium nitrite with 

 dithionite. 



Nitric oxide hemoglobin has a spectrum similar to that of oxyhemoglobin 

 and carboxyhemoglobin, but the bands are not so sharp, and the minimum 

 is not so well defined. It is found on magnetochemical investigation to have 

 one unpaired electron per iron atom {500) which must, however, be attributed 

 to the nitric oxide. Iron bonds are, therefore, covalent and of character 

 similar to those in oxyhemoglobin and carboxyhemoglobin. Nitric oxide is 

 even more firmly bound to hemoglobin than is carbon monoxide and will 

 therefore replace this molecule' from its linkage with hemoglobin. Even when 

 ferricyanide is present, which removes hemoglobin from the system, the 

 dissociation of HbNO proceeds extremely slowly {14S9). 



2. 2. .5. Cyanhemoglobin. Cyanhemoglobin is still a somewhat controversial 

 compound and it is doubtful if it has yet been obtained without admixture 

 of other pigments. In 1926, two groups of workers, Balthazar and Phillipe 

 (128) and Anson and Mirsky (cited in 2669) observed that, on the addition 

 of reducer to hemiglobin cyanide between pH 6 and 10, bands appeared at 

 561 and 533 mju and later slowly faded to the spectrum of hemoglobin. If 

 the cyanide concentration is increased, the two-banded spectrum is more 



