ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 137 



may constitute a real and important nuclear enzyme system, although 

 there is room for considerably more work on this subject. Hexokinase seems 

 to be absent from cell nuclei. 



h. Hydrolases. A number of hydrolytic enzymes have been reported in 

 cell nuclei. Arginase was the first enzyme to be found in isolated cell nuclei 

 (Behrens,*-). Arginase has also been found in high concentration in rat liver 

 cell nuclei isolated at pH 6 in very dilute citric acid^ or by a modification 

 of the Behrens technique.* This enzyme has also been found in mammalian 

 cell nuclei by Mirsky et al.,^^ who also used a modification of the Behrens 

 technique. The work of Dounce and G. T. Beyer' indicated that dissolved 

 arginase is not adsorbed to a sufficient degree by liver cell nuclei to account 

 for their high arginase activity. 



Although arginase is almost certainly a constituent of mammalian liver 

 cell nuclei, it is very scarce in or absent from isolated mammalian kidney 

 cell nuclei' '^^ and chicken liver nuclei, so that it is probably not generally 

 a nuclear constituent. 



Adenosinetriphosphatase is another hydrolytic enzyme thought by some 

 to be an important nuclear constituent, but this point is now in doubt. 

 Nuclear sediments obtained in sucrose (which were not of a high state of 

 purity) showed high adenosinetriphosphatase activity,"* as also did puri- 

 fied nuclei obtained by Lang et a/.,'-* who used strong sucrose solution as 

 the medium of isolation. Nuclei isolated at pH 6 in very dilute citric acid 

 show high adenosinetriphosphatase activity, '^^'^^ as also do those isolated 

 by the new method already described, involving the use of the ball-type 

 homogenizer and calcium chloride-sucrose solutions followed by adjust- 

 ment to pH 6.2 with citric acid.''" However, Mirsky et alP*'^ found little 

 adenosinetriphosphatase in nuclei isolated from various tissue by their 

 modification of the Behrens technique. 



In this case, it seems unlikely that contamination by adsorbed mito- 

 chondrial fragments could be causing the adenosinetriphosphatase activity 

 of the nuclei isolated in aqueous media, since use of the ball-type homoge- 

 nizer should effectively eliminate this source of error, and the situation may 

 be cited as an excellent illustration of the necessity for comparative studies 

 using nuclei isolated by different procedures. The nuclear adenosinetri- 

 phosphatase does not seem to be activated by dinitrophenol and might 

 conceivably be different from mitochondrial adenosinetriphosphatase. It is 

 suggested that no definite conclusions be drawn concerning the presence 

 of adenosinetriphosphatase in isolated nuclei until further work has been 

 done. 



Deoxyribonuclease is an interesting hydrolase concerning the intracellular 

 localization of which there is considerable disagreement. According to 



1" H. A. Lardy and Harlene Wellman, J. Biol. Chem. 201, 357 (1953). 



