624 G. SCHMIDT 



tation of the available data, however, requires caution because various 

 interfering factors must be considered. Some of those factors are: (1) The 

 presence of abundant amounts of substrates in some cell fractions, particu- 

 larly in studies concerning deoxyribonuclease and ribonuclease. (2) The pos- 

 sibility of absorption of enzymes by particles during homogenization. This 

 source of error is serious when citric acid procedures are employed for the 

 isolation of nuclei. (3) The influence of the ionic environment and other ac- 

 tivators and inhibitors on the action of the enzymes. 



1 . Deoxyribonuclease*^'' -"^ -^-^ ,277-279 



According to Allfrey and Mirsky,*^^ deoxyribonuclease I appears to be a 

 specific digestive enzyme of pancreas. It is not found in significant concen- 

 trations in other tissues, but its activity in pancreas (as differentiated by 

 means of its pH optimum) exceeds that of other deoxyribonucleases more 

 than 200-fold. It was found in high concentration in the pancreas fistula 

 juice of a dog, whereas no appreciable amounts of other deoxyribonucleases 

 could be demonstrated. 



Deoxyribonucleases of acid pH optima were found in many tissues in 

 which they are mainly associated with the cytoplasmic fraction, particularly 

 with the mitochondria (Chapter 18). In contrast to some earlier reports, it 

 appears now that the nuclei of liver, heart, and spleen have little if any de- 

 oxyribonuclease activity (see also Chapter 21). 



Kowlessar et al}^^-"^^^ found that considerable amounts of deoxyribonu- 

 cleases of acid as well as alkaline pH optima are excreted in the urine by 

 mice which had been irradiated with lethal doses of X-rays. This suggests 

 the liberation of intracellular nucleases under conditions producing exten- 

 sive cell damage. 



2. Ribonuclease 



According to Roth,^*^ ribonucleases of acid as well as alkaline pH optima 

 occur in many tissues although the concentration of ribonuclease I in pan- 

 creas exceeds by far the ribonuclease activity of any other animal tissue. 

 In liver, the mitochondrial fraction contains a larger part of "acid" and 

 "alkaline" ribonuclease activity than do other fractions of the homogenates. 

 Liver nuclei contain small, but measurable, amounts of ribonuclease activity 



2" W. C. Schneider and G. H. Hogeboom, J. Biol. Chem. 198, 155 (1952). 



"8 R. J. Neff, Thesis, Univ. of Missouri, 1951. 



"9 K. Land, G. Siebert, I. Baldus, and A. Corbet, Ex-perientia 6, 59 (1950). 



28" O. D. Kowlessar, K. I. Altman, and I. H. Hempelman, Arch. Biochevi. and Biophys. 



43, 233 (1953). 

 "1 O. D. Kowlessar, K. I. Altman, and I. H. Hempelman, Nature 172, 867 (1953). 

 "2 J. S. Roth, /. Biol. Chem. 208, 181 (1954). 



