BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 501 



very likely that nuclear PXA is not the sole precursor of cytoplasmic PXA, 

 even though the presence of the nucleus is necessary for the prolonged 

 maintenance of microsomes in enucleated organisms: recent experiments, 

 made in this laboratory, show conclusively that the nonnucleated halves 

 of certain unicellular organisms still incorporate orotic acid readily into 

 their PNA fraction; but whether we are dealing with a true synthesis or 

 a mere turnover is not yet known. 



After this discussion on the role of the nucleus in PXA metabolism, we 

 shall turn to another aspect of Caspersson's theory i^*'***'^"^ is the nucleus 

 "a cell organelle organized especially for being the main center of the cell 

 for the formation of proteins"? 



(3) The Role of the Nucleus in Protein Synthesis. Isotope studies by 

 Daly et al.^^* on incorporation of glycine-X'^ in isolated nuclear and cyto- 

 plasmic proteins have shown that speed of incorporation varies considera- 

 bly according to the tissue used; but incorporation always follows the same 

 pattern, being high in both nuclear and cytoplasmic proteins in actively 

 metabolizing tissues, such as liver or pancreas, and low in sperm or chicken 

 erythrocytes. Among the nuclear proteins, the "residual proteins" show 

 a more rapid uptake than the histones; the rate of incorporation in these 

 residual proteins in liver is about the same as for mixed cytoplasmic pro- 

 tein, while it is lower in pancreas. 



These results of Daly et al.^^* have been confirmed, for liver, by Smellie 

 et al.,^-'' working with formate-C", methionine-S^% and glycine-X^^: the 

 incorporation of all of these labeled substances into nuclear proteins has 

 been found to be of the same magnitude as into the cytoplasmic proteins. 



All these observations deal with mixed cytoplasmic proteins, i.e., the 

 proteins of whole cytoplasm which has not been subjected to fractionation 

 by differential centrifugation ; results obtained on the incorporation of 

 amino acids into the various cytoplasmic cell particles will be discussed 

 later. 



It is obvious from the existing data that the incorporation of labeled 

 amino acids is no higher in the nucleus than it is in the cytoplasm, as might 

 be expected if the nucleus were the main center of protein synthesis. It 

 should, however, be emphasized that we know as yet nothing about the 

 relative rates of incorporation of labeled amino acids into the nucleus and 

 into the cytoplasm in tissues endowed with a high mitotic activity. 



The capacity for nonnucleated cells such as reticulocytes to incorporate 

 amino acids into their proteins is now a well-established fact: after the 

 initial demonstration by London et al}*'" that reticulocytes, in contrast to 

 mature erythrocytes, can incorporate labeled glycine into the heme moiety 



1" M. M. Daly, V. G. Allfrey, and A. E. Mirsky, J. Gen. Physiol. 36, 173 (1952). 

 i« I. M. London, D. Shemin, and D. Rittenberg, /. Biol. Chem. 183, 749 (1950). 



