EXPERIMENTAL RESULTS 107 



might well be present in lysosomes, these baglike granules which 

 apparently contain enzymes in solution, according to De Duve 

 et al. (1955). Unequal resistance of the lysosomes to autolytic pro- 

 cesses induced by the removal of the nucleus might explain all the 

 results ; such a hypothesis would also help to explain the temporary 

 increase in activity of amylase in the enucleate halves. Nothing 

 more definite than that can be said until more is known about the 

 intracellular distribution of the hydrolytic enzymes in amoebae. 



Recent experiments in the author's laboratory by Sells and Six 

 (1959) on adenosine triphosphatase (ATPase) clearly show how 

 complicated the situation can be. They confirmed our earlier finding 

 that the activity of total ATPase remains unchanged after removal 

 of the nucleus, even after 8 days. But they also followed the activity 

 of the "ecto-ATPase", that is, the enzyme which is present in the 

 membrane and which can be estimated by following the break- 

 down of ATP added to the medium. For normal amoebae and 

 for nucleate fragments, identical values were obtained for total 

 and ecto- ATPase ; but, in the enucleate fragments, the ecto-ATPase 

 activity decreased markedly (about 50 % in 5 days) although, as 

 mentioned above, the total ATPase activity remained unchanged. 

 It might be that part of the enzyme normally localized on the mem- 

 brane shifts to the interior of the cell upon removal of the nucleus. 



Protein metabolism in fragments of amoebae has, of course, also 

 been analyzed by studies on the incorporation of labeled amino 

 acids. Mazia and Prescott (1955) carefully studied the incorporation 

 of ^^S-methionine into the proteins of the two halves and found that 

 the percent incorporation is lower by a factor of 2-5 in the enucleate 

 half immediately after the cell has been cut into two. The ratio of 

 the incorporation in the nucleate and the enucleate halves rose to 

 values as high as 20 after a few days. Mazia and Prescott (1955) 

 concluded that the nucleus is either the seat of a considerable pro- 

 portion of the protein synthesis or that this nucleus-linked syn- 

 thesis is very closely coupled with processes that are localized in the 

 nucleus. But the experiments also show that the nucleus is not the 

 exclusive center of protein synthesis in the cell. 



Autoradiographic experiments of Ficq (1956) and Brachet and 



References p. 133/ J 35 



