500 J. BRACKET 



sion is also borne out by the-^-ecent experiments of Hogeboom and Schnei- 

 der,'^^ showing that the enzyme catalyzing the synthesis of DPN from 

 nicotinamide mononucleotide and ATP is largely, if not exclusively, 

 confined to the cell'imcleus in the liver. 



Finally, another type of experiment of a more biological nature leads to 

 the same conclusion. When amoebae (Amoeba proteus) are cut into two 

 parts, only one part retains the single nucleus. In the absence of any exter- 

 nal food supply, the nucleated half survives for about 3 weeks, while the 

 nonnucleated part dies after 2 weeks. Cytochemical staining methods 

 have shown that the basophilia of the nonnucleated half drops markedly 

 3 to 4 days after the operation and that it becomes very weak after 10 

 days (Brachet'^"'^'): as early as the 4th or 5th day after enucleation, the 

 difference between the two types of fragments is very striking. Micro- 

 chemical analysis of the PNA content of both halves has yielded results 

 which are consistent with the cytochemical data (Brachet,''*' Linet and 

 Brachet'^^) : a distinct drop in the PNA content of the nonnucleated halves 

 is already apparent on the 3rd day after the operation. Ten days after 

 enucleation, the nonnucleated parts contain, on the average, only one- 

 third of their initial PNA. 



There is no doubt that the removal of the nucleus is followed by a rapid 

 and considerable drop of the PNA content in the cytoplasm: cytoplasmic 

 PNA is unquestionably dependent on the nucleus for its prolonged main- 

 tenance. Similar conclusions can be drawn from the study of other cell 

 types, e.g., maturing red blood cells: it is a well-known fact that reticulo- 

 cytes represent an intermediary stage in hemopoiesis during which the 

 cytoplasm still retains PNA although the nucleus has already been lost; but 

 the reticulocytes are soon converted into mature erythrocytes, which have 

 almost no PNA in their cytoplasm (Thorell,^^ Holloway and Ripley''*'). 



Summing up, it might be said that there is at present good evidence that 

 the cell nucleus plays a very important role in PNA and nucleotide metab- 

 olism: all the facts known, whether they refer to isotope studies, intra- 

 cellular distribution of enzymes, or behavior of enucleated organisms, point 

 in the same direction. On the other hand, whether nuclear PNA is a direct 

 precursor of cytoplasmic PNA cannot yet be decided with certainty: 

 although many facts are compatible with such a hypothesis, it is highly 

 probable that, if such a transfer of nuclear PNA to the cytoplasm occurs, 

 it is a more complex phenomenon than simple diffusion. Furthermore, it is 



139 G. H. Hogeboom and W. C. Schneider, J. Biol. Chem. 197, 611 (1952). 



"« J. Brachet, Experientia 6, 56 (1950). 



i« J. Brachet, Symposia Soc. Exptl. Biol. 6, 173 (1952). 



i« N. Linet and J. Brachet, Biochim. et Biophys. Acta 7, 606 (1951). 



1" B. W. Holloway and S. H. Ripley, /. Biol. Chem. 196, 695 (1952). 



