KINETICS OF HEMOGLOBIN REACTIONS 281 



6.2.3. CO + Hb — ^ HbCO. This reaction is much slower than the 

 corresponding reaction with oxygen but is of the same type (2356). 

 Up to 50% carboxyhemoglobin, the reaction is described by the 

 equation : 



d[UhCO]'dt = ko [Hb] [CO] - ki [HbCO] 



That the reaction is better described by this equation than by Hill's 

 can be seen from the fact that 32-fold variation of the ratio [CO] /[Hb] 

 varies the rate proportionally and not 32" times as would be expected 

 on Hill's theor\\ Between />H 5.6 and 7.5, the rate remains unaltered, 

 but is 50% faster at pH 10. The temperature coefficient Qio is 

 approximately 2. Hundred-fold variation in the intensity of the 

 light did not affect the reaction, an indication that the back reaction 

 was negligible. The effect varying salt concentration might have on 

 the reaction was not investigated. 



6.2.4. HbCO — ^ CO + Hb. As has been well known, this is the 

 slowest of the four reactions so far investigated, and, since the disso- 

 ciation is slower than the association reaction, it is no longer per- 

 missible to neglect the second term in the equation: 



- f/[HbCO] (It - ki [HbCO] - ko [Hb] [CO] 



Furthermore, it is impossible to utilize the same technique as was 

 used in the measurement of the dissociation of oxyhemoglobin, no 

 reagent being known which will combine with carbon monoxide 

 without affecting the protein. 



Roughton {2357) considered making use of the reaction: 



Hb + [FefCX)^]' - -^ Hi + [FefCX)6p " 



to remove the hemoglobin from the system as it was formed from carboxy- 

 hemoglol)in. This should be compared with the removal of oxygen by means 

 of dithionite in tlie analogous oxyhemoglobin reaction. The velocity found 

 in using this device was however some oO% greater than that found by the 

 method described in the succeeding section in which oxygen was used to 

 combine with the hemoglobin. 



While the dissociation of the first carbon monoxide molecule occurs under 

 the same conditions in both methods, subsequent molecules dissociate from 

 a different structure in the presence of ferricyanide; the structures: 



, . CO . 

 V 1 cck m I 1 4- 



