OSWALD T. AVERY, COLIN M. MACLEOD, AND MACLYN MCCARTY 147 



preparations shown to contain an enzyme capable of depolymerizing authentic 

 samples of desoxyribonucleic acid were found to inactivate the transforming 

 principle. 



Greenstein and Jenrette (18) have shown that tissue extracts, as well as the 

 milk and serum of several mammalian species, contain an enzyme system which 

 causes depolymerization of desoxyribonucleic acid. To this enzyme system 

 Greenstein has later given the name desoxyribonucleodepolymerase (19). 

 These investigators determined depolymerase activity by following the reduc- 

 tion in viscosity of solutions of sodium desoxyribonucleate. The nucleate 

 and enzyme were mixed in the viscosimeter and viscosity measurements made 

 at intervals during incubation at 30°C. In the present study this method was 

 used in the measurement of depolymerase activity except that incubation was 

 carried out at 37°C. and, in addition to the reduction of viscosity, the action 

 of the enzyme was further tested by the progressive decrease in acid precip- 

 itability of the nucleate during enzymatic breakdown. 



The effect of fresh normal dog and rabbit serum on the activity of the 

 transforming substance is shown in the following experiment. 



Sera obtained from a normal dog and normal rabbit were diluted with an equal 

 volume of physiological saline. The diluted serum was divided into three equal 

 portions. One part was heated at 65°C. for 30 minutes, another at 60°C. for 30 

 minutes, and the third was used unheated as control. A partially purified prepara- 

 tion of transforming material which had previously been dried in the lyophile appara- 

 tus was dissolved in saline in a concentration of 3.7 mg. per cc. 1.0 cc. of this solution 

 was mixed with 0.5 cc. of the various samples of heated and unheated diluted sera, 

 and the mixtures at pH 7.4 were incubated at 37°C. for 2 hours. After the serum had 

 been allowed to act on the transforming material for this period, all tubes were heated 

 at 65°C. for 30 minutes to stop enzymatic action. Serial dilutions were then made in 

 saline and tested in triplicate for transforming activity according to the procedure 

 described under Method of titration. The results given in Table III illustrate the 

 differential heat inactivation of the enzymes in dog and rabbit serum which destroy 

 the transforming principle. 



From the data presented in Table III it is evident that both dog and rabbit 

 serum in the unheated state are capable of completely destroying transforming 

 activity. On the other hand, when samples of dog serum which have been 

 heated either at 60°C. or at 65°C. for 30 minutes are used, there is no loss of 

 transforming activity. Thus, in this species the serum enzyme responsible 

 for destruction of the transforming principle is completely inactivated at 

 60°C. In contrast to these results, exposure to 65°C. for 30 minutes was 

 required for complete destruction of the corresponding enzyme in rabbit serum. 



The same samples of dog and rabbit serum used in the preceding experiment 

 were also tested for their depolymerase activity on a preparation of sodium 

 desoxyribonucleate isolated by Mirsky from shad sperm. 



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