394 R. M. S. SMELLIE 



VII. The Catabolism of the Nucleic Acids 422 



1. General 422 



2. The Degradation of the Purine Bases 423 



a. The Formation and Breakdown of Uric Acid 423 



b. The Metabolism of Uric Acid 426 



3. The Catabolism of the Pyrimidines 427 



VIII. Addendum 429 



I. Introduction 



Between ribonucleic acid (PNA) and deoxyribonucleic acid (DNA) 

 there is a major difference in metabolic properties; the DNA remains rela- 

 tively constant in amount throughout the greater part of the life of the 

 cell while the amount of PNA may vary considerably. This suggests that 

 DNA may be a comparatively inert component, while PNA is actively 

 engaged in the metabolic processes of the cell. While this may be true of 

 resting cells, rapid synthesis of DNA must necessarily take place during 

 division if one cell is to give rise to two, each with the same DNA content 

 as the parent (Chapter 19). It may be expected therefore that PNA will 

 prove to be metabolically active even in resting tissues, but that DNA will 

 be relatively inert in resting tissues and more active in proliferating tis- 

 sues such as intestinal mucosa, bone marrow, and regenerating liver. 



The advent within recent years of isotope techniques in biological re- 

 search has overcome many of the difficulties which confronted earlier 

 workers on the metabolism of the nucleic acids, and the way has been 

 opened to studies on their biosynthesis, metabolism, and catabolism. The 

 aspects of nucleic acid metabolism associated with biosynthesis have 

 been discussed in the preceding chapters of this volume, and this chapter 

 will be restricted as far as possible to a discussion of the metabolic activity 

 and catabolism of the nucleic acids. 



II. The Use of P^^ in Studies on Nucleic Acid 

 Metabolism 



1. DNA 



a. The Relative Activities of the DNA's from Different Tissues 



Radioactive phosphorus has been widely used in studies on the me- 

 tabolism of the DNA from different tissues. Some of the results obtained 

 are illustrated in Table I (see also Table II in Chapter 25). From these fig- 

 ures, which are expressed as ratios relative to the activity of the spleen DNA 

 in each group, it is evident that the tissues fall into two main classes with 

 respect to incorporation of P^^ into the DNA : those such as bone marrow, 

 spleen, thymus, and intestinal mucosa, which may be regarded as pro- 

 liferating tissues, in which uptake of isotope is high; and the remainder 



