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R. W. Henderson and W. A. Rawlinson 



n which is between 1 and 2 at the commencement of titration, approaches 2 

 as the reduction continues, and after the point of 50 % reduction is quite 

 close to 2. These changes would appear to provide an explanation of the 

 altered biological activity of this fraction. 



20 40 60 80 100 



Reduction, % 



Fig. 6. Oxidation-reduction titration at pH 6-4 of 'modified' ox-heart cyto- 

 chrome c (Fraction II according to Margoliash, 1954b). Observed points •; 

 theoretical curves are for E^ = -fO-310 V; broken line, « = 1 ; solid line, « = 2. 

 The oxidation-reduction potential of 'unmodified' fraction {Eq = +0-255 V) is 

 shown thus O. 



Margoliash (1954a, b) obtained evidence that in the case of horse-heart 

 cytochrome c, TCA at pH 4-5 was responsible for the formation of Fraction 

 II above. We subjected resin-purified samples from ox-heart to treatment 

 with TCA at approx. pH 1, (2-5% TCA, 18 hr, 25°C) and obtained a similar 

 E'q value to that of Fraction II. Again there was evidence of aggregation 

 although in this case it was not as pronounced as in Fraction II above; the 

 curve was in between n = 1 and n = 2 after 50% reduction. It is of con- 

 siderable interest that the ascorbic acid oxidase activity remained virtually 

 unaltered (Fig. 2). It did not behave on the resin column like Fraction II 

 as it was eluted slowly and diffusely by 0-25 m ammonium acetate at pH 7. 



