13] STRUCTURE & ENZYMATIC ACTIVITY OF RIBONUCLEASE 237 

 had an inactivating effect over and above that shown by equivalent levels 

 of sodium chloride. The data in Fig. 10 show that guanidinium ions exert 

 a specific inactivating effect, not attributable to ionic strength, in the con- 

 centration range between 1 and 3 molar. It is precisely in this range of 

 concentrations that low levels of orthophosphate no longer prevent the shift 





-250 -200 -150 -100 -50 



[q]x 



Fig. 9. The optical rotatory dispersion of ribonuclease in : (A), water, (B), 6m guanidine 

 hydrochloride containing 0-3m phosphate buffer, pH 6-5, (C), 6m guanidine hydrochloride's 

 The data are plotted according to the suggestion of Yang and Doty.^* 



MOLARITY OF SALT 



Fig. 10. Activity of ribonuclease in sodium chloride solutions (solid circles), guanidine 

 hydrochloride solutions (open circles). Activity measured by alkali uptake in the Jacobsen- 

 Lconis^^ autotitrator at pH 6 and plotted as the velocity constant, k, of the first order 

 reaction, in minutes"^.'^ 



in spectrum characteristic of the inactive enzyme. We may conclude, there- 

 fore, that the inactivation of ribonuclease by guanidinium ions is also closely 

 correlated with a change in the ultraviolet absorption properties of certain 

 tyrosine residues. The guanidine effect is completely reversible upon dilu- 

 tion of the system or following removal of this material by dialysis. 



