Modification of the Secondary Structure of Haemoprotein Molecules 81 



on addition of EIC. Figure 1 shows the absorption spectra of haemo- 

 globin in the presence of EIC at various concentrations. Clear isosbestic 

 points at 542, 549 and 568 m/f were demonstrated indicating that the reaction 

 system contained only two haemoglobin components. Figure 1, K represents 

 the absorption spectrum of fully saturated ElC-haemoglobin. Taking the 

 absorption spectrum of Fig. 1 , K as that of the saturated ElC-haemoglobin, 



550 



m/i. 



500 



Fig. I . Absoqjtion changes of reduced haemoglobin at various ethyl isocyanide 

 (EIC) concentrations. Haemoglobin concentration, 30 x 10"^ mole/1., and EIC 

 concentrations: I, zero; II, 4-85 x IQ-^m; III, 7-78 x IQ-^m; IV, 1-32 x 

 10-* m; V, 1-32 x 10-3 j^ ^^^ 2-64 x 10"^ m. Spectra were measured at pH 6-8 



and at 25°C. 



the percentual formation of ElC-haemoglobin at each EIC concentration 

 can be calculated. 



The plot of these values against the log concentrations of uncombined EIC 

 gave rise to a sigmoid curve of a high order reaction shown in Fig. 2. The 

 sigmoid coefficient n of the symmetrical sigmoid curve in Fig. 2 was calculated 

 to be 2-4, by introducing the data obtained in the present experiment into 

 Hill's equation (Hill, 1910) 



Y = V/1 + ^/ 



which has been proposed for the reaction of reduced haemoglobin with 

 oxygen. It is thus proved that there exists a haem-haem interaction also in 

 the reaction of reduced haemoglobin with EIC, the same as in the reaction of 

 haemoglobin with oxygen. The sigmoid coefficient of the reaction of reduced 

 haemoglobin with oxygen has been reported to be 2-8 (Wyman, 1948). 



