THE NUCLEIC ACIDS 



It was crystallized by Kunitz in 1948. McCarty developed a rrfethod 

 for estimating the activity of a preparation of the enzyme, by making 

 use of the high viscosity of sodium thymonucleate in solution. The rate 

 of decrease is measured when the substrate is incubated with de- 

 oxyribonuclease. By this method, it was shown that the enzyme is 

 activated by magnesium ions. One group of inhibitors are substances 

 which remove this ion (Figure 2). 



Numerous papers have been published on the cytochemical use of 

 these enzymes. It is necessary to incubate a control series of sections in 

 the buffer solution used to dissolve the enzyme, and then to compare the 

 sections exposed to the latter with those treated with buffer alone. 

 After ribonuclease digestion, either basophilia or absorption in ultra- 



3-0 



2-0 



1-0 



10 



20 30 



Time 



VO 



50 



Figure 2 Relative viscosity curves of 

 DNA a in the presence of desoxyribonu- 

 clease and o- 002 m MgS04 Magnesium, 

 b with 0002 M Sodium arsenate, c with 

 0'002 M Sodium citrate. {By courtesy of 

 Dr Michael Webb.) 



violet light is compared in the two series of preparations. Among the 

 earliest papers in this field are those of Bracket^* in 1941, who used 

 the methyl green-pyronin staining method of Unna-Pappenheim as 

 the test for the effects of the enzyme. Bracket found that in a number 

 of cells and tissues, such as exocrine glands and oocytes in which there 

 occurs a rapid synthesis of material, their basophilia was much re- 

 duced by treatment with ribonuclease. 



Davidson and Waymouth showed that in the cytoplasm of rat liver 

 cells, digestion with ribonuclease decreased both the affinity for tolui- 

 dine blue^^ and the absorption of ultraviolet light.^^ In the nucleolus, 

 the results were partly masked by the outer zone of heterochromatic 

 deoxynucleoprotein. Although there are a number of apparently 

 successful instances of the cytochemical use of ribonuclease, some 

 authors including Davidson^' himself have drawn attention to diffi- 

 culties in its use. Stowell and Zorzoli^^ have shown how much the 



