ENZYMES ATTACKING NUCLEIC ACIDS 579 



In deoxyribonuclease digests of calf thymus DNA, thymidylic, cytidylic, 

 adenylic, and guanylic acids were detected; in those of wheat germ DNA, 

 methylcytidylic acid was found in addition to the other mononucleotides. 

 The amounts of thymidylic acid accounted for 50 to 60% of the total 

 mononucleotides formed. 



Dinucleotides accounted for 15 to 18% of the substrate phosphorus. 

 Although most of the dinucleotides contained a pyrimidine group, appre- 

 ciable amounts of purine dinucleotides were found in deoxyribonuclease 

 digests of calf thymus and wheat germ DNA. 



From the fact that exclusively 5 '-mononucleotides were obtained by 

 the action of phosphodiesterase^^ on oligonucleotides obtained with deoxy- 

 ribonuclease I, one can imply that the terminal secondary phosphoryl 

 groups of these oligonucleotides are attached to the 5 '-positions of the 

 corresponding deoxyribose moieties. 



An appreciable portion of the higher oligonucleotides of deoxyribonu- 

 clease I digests is not dialyzable against water. This undialyzable fraction 

 was designated by Zamenhof and Chargaff*® as "cores." These cores have 

 much higher adenine: guanine, thymine :cytosine, and purine: pyrimidine 

 ratios than are found in the original substrate. The possibility of a correla- 

 tion between the sequence of the bases in DNA and the points of attack 

 of deoxyribonuclease is not yet clear, but it appears that some seeming 

 resemblance of deoxyribonuclease and ribonuclease action in this respect 

 does not justify the assumption of similar underlying specificities. 



The effect of deoxyribonuclease on the viscosity of DNA has recently 

 been used for some therapeutic purposes. ^""^ 



Assay Methods. It is obvious that some of the tests for deoxyribonuclease 

 activity resemble in principle those for ribonuclease activity and need not 

 be discussed in detail. Formation of acidic groups: The transformation of 

 primary into secondary phosphoryl groups can be measured by titration 

 or manometric methods. The conditions for such measurements with DNA 

 as substrate are more favorable than similar determinations of ribonuclease 

 activity because of the negligible amounts of preformed secondary phos- 

 phoryl groups in highly polymerized DNA. Formation of acid-soluble 

 phosphorus or deoxyribose compounds: The merits of these assay methods 

 have been discussed by Laskowski,''^ Kurnick,*^ and Allfrey and Mirsky.^"^ 

 The determination of acid-soluble P compounds in deoxyribonuclease 



8« S. Zamenhof and E. Chargaff, J. Biol. Chem. 187, 1 (1950). 

 86" J. N. Davidson, Brit. Med. Bull. 9, 154 (1953). 

 " N. B. Kurnick, Arch. Biochem. 29, 41 (1950). 

 s"* V. G. Allfrey and A. E. Mirsky, J. Gen. Physiol. 36, 227 (1952). 

 "■^ S. G. Laland, W. A. Lee, W. G. Overend, and A. R. Peacocke, Biochim. el Biophys. 

 ^c<a 14, 356 (1954). 



