578 G. SCHMIDT 



or total removal of the purine groups, however, greatly diminishes the 

 catalytic effects of the enzyme. It should be mentioned that Tamm et al. 

 observed in the absence of deoxyribonuclease I degrading effects of the 

 bivalent cations (Mg++, Mn++) required as activators of deoxyribonuclease 

 I on the deoxypolynucleotides of lower molecular weights. These effects 

 obviously render it difficult to evaluate accurately the action of deoxyribo- 

 nuclease on DNA fragments of lower molecular weights. Little and Butler^^ 

 isolated the nucleotide fraction formed by exhaustive enzymic degradation 

 of highly polymerized (acid-insoluble) DNA to acid-soluble nucleotides. 

 The titration of this fraction demonstrated the formation of secondary 

 phosphoryl groups amounting to 25 % of the total phosphoryl groups. This 

 result is in agreement with earlier observations of Fischer, Bottger, and 

 Lehmann-Echternacht.^" Gordon and Reichard,^^ Sinsheimer and Koer- 

 ner,^2 ,83 ,83a as y^Qii as Smith and Markham^^ have succeeded recently in 

 identifying by chromatography on paper and on ion-exchange columns 

 some of the oligonucleotides as dinucleotides and trinucleotides. So far, 

 deoxyadenyl-cytidylic, adenyl-thymidylic, guanyl-thymidyhc, thymidylyl- 

 thymidylic, cytidylyl-cytidylic, methylcytidylyl-cytidylic, methylcytidy- 

 lyl-guanylic, adenyl-adenylic, adenyl-guanylic, guanyl-guanylic acids and 

 a trinucleotide consisting of one thymidylic and two cytidylic acid groups 

 were identified amongst the degradation products present in deoxyribonu- 

 clease digests of calf thymus DNA. Digests of wheat germ DNA contained 

 methylcytidylyl-cytidylic and methylcytidylyl-adenylic acids in addition 

 to the dinucleotides just mentioned. It is interesting that not all dinucleo- 

 tides which have been analyzed up to now contain a pyrimidine component 

 in contrast to the oligonucleotides formed from PNA by the action of 

 ribonuclease. The amounts of mononucleotides formed by the action of 

 deoxyribonuclease on DNA are much smaller than those formed by ribo- 

 nuclease from PNA. The first- evidence for the formation of mononucleo- 

 tides by the hydrolysis of DNA with deoxyribonuclease I was obtained 

 by Potter, Brown, and Laskowski,^^ who detected the appearance of 

 appreciable quantities of deoxycytidylic acid in such digests. They repre- 

 sent only approximately 1 % of the total phosphorus of the substrate.*^^ 



79 J. A. Little and G. C. Butler, J. Biol. Chem. 188, 695 (1951). 



ST. G. Fischer, I. Bottger, and H. Lehmann-Echternacht, Z. physwl. Chem. 271, 

 246 (1941). 



81 A. H. Gordon and P. Reichard, Biochem. J. 48, 569 (1951). 



82 R. L. Sinsheimer and J. F. Koerner, Science 114, 42 (1951). 



83 R. L. Sinsheimer and J. F. Koerner, /. Am. Chem. Soc. 74, 283 (1952). 

 83» R. L. Sinsheimer, J. Biol. Chem. 208, 445 (1954). 



84 J. D. Smith and R. Markham, Biochim. et Biophys. Ada 8, 350 (1952). 



85 J. L. Potter, K. D. Brown, and M. Laskowski, Biochim. et Biophys. Acta 9, 150 

 (1952). 



