284 VI. HEMOGLOBIN 



These results do not present the same paradox as is presented by 

 the agreement between the equiHbrium constant as found for the 

 reaction HbOa ^ Hb + O2 and that calculated from the velocity 

 constants of the forward and back reactions (Section 6.2.2.), since 

 the Hiifner equation is known to describe the behavior of the system 

 CO + HbCO ^ HbCO + O2. 



6.2.7. Kinetics of Reactions within the Erythrocyte- In several papers 

 Roughton {2355,2363) has approached the problem of the effect of the 

 erythrocyte on the kinetics of the hemoglobin reactions. The influence of 

 diffusion is not noticeable when slow reactions such as the dissociation of 

 carboxyhemoglobin are measured, the rate of which appears to be the same 

 in the erythrocyte as in solutions of hemoglobin. With faster reactions 

 diffusion rates through the cell membrane and through the interior of the 

 cell become progressively more important as limiting factors. Thus in the 

 combination of carbon monoxide with hemoglobin the rate is about one-third 

 as fast in the cell as in solution, and in the fastest reaction of all, the com- 

 bination of oxygen with hemoglobin, the rate is reduced to one-twelfth. At 

 partial pressures of oxygen above 400 mm. oxygen displaces carbon monoxide 

 from carboxyhemoglobin as fast in cell suspensions as in solutions. If the 

 value of k2 (cf. Section 6. 2. .5.) was increased by further lowering the concen- 

 tration of oxygen, Roughton considered that the velocity of diffusion into the 

 cell would be detectable, so that at alveolar oxygen pressures the rate in the 

 cell would be 87% of that in solution. So far, however, experimental con- 

 firmation of this figure is lacking {cf. Chapter VII, Sections 7.2.3. and 11.3.) 



6.2.8. Kinetics of Myohemoglobin Reactions. This pigment has 

 been less investigated than has hemoglobin. In particular, knowledge 

 is lacking of the proton dissociation of myohemoglobin in the neigh- 

 borhood of 7>H 7 when oxygen enters the molecule, although the 

 insensitivity of the dissociation curve to changes in pH suggests that 

 no ionization occurs. 



The gaseous reactions of myohemoglobin were among the first 

 investigated by the syringe modification of the Hartridge and Rough- 

 ton rapid flow technique {2368). The reaction mechanisms were 

 investigated in the usual way by altering the concentrations of the 

 different components in the system and the essential data {1952,1953) 

 are shown in Table IV {cf. Section 7.2.). 



Since myohemoglobin contains only one heme, certain variations of the 

 methods adopted for hemoglobin were possible in the study of the myohemo- 

 globin reactions. Thus, Millikan's use of ferricyanide to remove the myo- 

 hemoglobin formed by the dissociation of myocarboxyhemoglobin cannot 

 introduce undesirable heme-heme interactions as in the case of the hemo- 

 globin system. In the analogous di.ssociation reaction with oxyhemoglobin. 



