ENZYMES ATTACKING NUCLEIC ACIDS 577 



what earlier, Schmidt^^ had ah-eady found that fresh extracts of nucleo- 

 histone from thymus glands were not hydrolyzed by intestinal phosphatase, 

 but that they were dephosphorylated by this enzyme after a preceding 

 incubation with crude trypsin. This effect was undoubtedly due to the 

 enzymic depolymerization of the nucleic acid component of the nucleo- 

 histone by the deoxyribonuclease present in the trypsin preparations, but 

 not to proteolysis. Laskowski^^ (1946) and McCarty" achieved a consider- 

 able purification of the enzyme obtained from pancreas. The most essential 

 contribution of this work was the separation of deoxyribonuclease from 

 the powerful ribonuclease of pancreas and the evidence for the strict 

 specificity of deoxyribonuclease for DNA. The crystallization of the enzyme 

 which precipitates at much lower ammonium sulfate concentration (0.4 

 sat.) than does ribonuclease I (0.8 sat.) was achieved by Kunitz^'' in 1950. 



An interesting historical fact is the influence which the now abandoned 

 working hypothesis of the tetranucleotide structure of nucleic acids had on 

 the interpretation of the action of deoxyribonuclease. All investigators 

 agreed that, after exhaustive incubation of DNA with deoxyribonuclease, 

 about one out of four phosphoryl groups is present as a terminal secondary 

 phosphoryl group. For a long time this observation was considered as 

 evidence suggesting that the main products of the enzymic degradation 

 were tetranucleotides and that, therefore, natural DNA should be con- 

 sidered as polymers of tetranucleotide units." This view was predominant 

 until, in 1951, chromatography was developed by Cohn and Volkin, and 

 Smith and Markham, as a tool for the fractionation of oligonucleotide 

 mixtures. We know now that there is no correlation between the relative 

 amount of terminal phosphoryl groups and the chain length of the oligo- 

 nucleotide units, and that deoxyribonuclease digests of DNA contain 

 dinucleotides as well as hexanucleotides and oligonucleotides of other 

 degrees of polymerization. 



Specificity. The number of substrates tested for their behavior toward 

 deoxyribonuclease is far less extensive than that studied with ribonuclease I. 

 Nevertheless, the available observations justify the conclusion that deoxy- 

 ribonuclease is a highly specific phosphodiesterase; in particular, it is 

 without any effect on ribonucleic acids and ribopolynucleotides. In analogy 

 to the action of ribonucleases, that of deoxyribonuclease I never results in 

 the formation of inorganic phosphate. According to Tamm, Shapiro, and 

 Chargaff,^'" the degree of polymerization of DNA is not of essential influence 

 on the rate and on the extent of the action of deoxyribonuclease; partial 



7*G. Schmidt, Enzymologia 1, 135 (1936). 



'« M. Laskowski, Arch. Biochem. H, 41 (1946). 



" M. McCarty, J. Gen. Physiol. 32, 39 (1948). 



" C. Tamm, H. S. Shapiro, and E. Chargaff, J. Biol. Chem. 199, 313 (1952). 



