20 CHEMICAL SPECIFICITY OF NUCLEIC ACIDS 



deoxypentose nuclease of yeast cells^^-^^ xhis investigation af- 

 forded a possibility of exploring the mechanisms by which an 

 enzyme concerned with the disintegration of deoxypentose 

 nucleic acid is controlled in the cell. Our starting point again was 

 the question of the specificity of deoxypentose nucleic acids; but 

 the results were entirely unexpected. Since we had available a 

 number of nucleic acids from different sources, we wanted to 

 study a pair of deoxypentose nucleic acids as distant from each 

 other as possible, namely that of the ox and that of yeast, and 

 to investigate the action on them of the two deoxypentose 

 nucleases from the same cellular sources. The deoxyribonuclease 

 of ox pancreas has been thoroughly investigated, as was men- 

 tioned before. Nothing was known, however, regarding the exist- 

 ence of a yeast deoxypentose nuclease. 



It was found that fresh salt extracts of crushed cells contained 

 such an enzyme in a largely inhibited state, due to the presence 

 of a specific inhibitor protein. This inhibitor specifically inhibited 

 the deoxypentose nuclease from yeast, but not that from other 

 sources, such as pancreas. The yeast enzyme depolymerized the 

 deoxyribose nucleic acids of yeast and of calf thymus, which dif- 

 fer chemically, as I have emphasized before, at about the same 

 rate. In other words, the enzyme apparently exhibited inhibitor 

 specificity, but not substrate specificity. It is very inviting to as- 

 sume that such relations between specific inhibitor and enzyme, 

 in some ways reminiscent of immunological reactions, are of 

 more general biological significance. In any event, a better under- 

 standing of such systems will permit an insight into the delicate 

 mechanisms through which the cell manages the economy of its 

 life, through which it maintains its own continuity and protects 

 itself against agents striving to transform it. 



11. CONCLUDING REMARKS 



Generalizations in science are both necessary and hazardous; 

 they carry a semblance of finality which conceals their essentially 

 provisional character; they drive forward, as they retard; they 



