CHAPTER 25 

 ELEMENTS CONSTITUTING MAJOR ORGANIC METABOLITES 



Ellsworth C. Dougherty 



25.1. Introduction. For purposes of this volume the major elements con- 

 stituting metabolites are considered to be the following six: hydrogen, carbon, 

 nitrogen, oxygen, sulfur, and phosphorus. These are the fundamental 

 elements of the various proteins, which form the framework of all living cells. 

 Useful tracer isotopes exist for all six, and at least one isotope has already been 

 applied for each. It may be noted that all these elements play a role in 

 mineral metabolism, but this is of secondary importance to their organic 

 function. 



Since the pioneer work of Schoenheimer who initiated the use of isotopes 

 in the study of intermediary organic metabolism slightly more than a decade 

 ago, a very large literature has grown up. The most important generalizing 

 concept that has come out of this work has been that of the dynamic state, 

 or equilibrium of all organic body constituents [Genll2], Work on inter- 

 mediate metabolism of organic substances — carbohydrates, fats, proteins, 

 etc. — has shown that all are undergoing a continuous process of breakdown 

 and resynthesis, even seemingly inert substances such as stored body fat. 



The application of isotopes has already solved many important problems 

 in the metabolism of organic substances, but the future unquestionably holds 

 a far vaster range of application. 



25.2. Carbon. Carbon, which forms the "backbone" of all organic com- 

 pounds, has three useful isotopes for tracer work: the rarer stable species C 13 

 and two unstable species C u and C 14 . In application, C 11 is limited to short- 

 term experiments by reason of its short half-life (20.5 min), whereas C 13 

 (stable) and C u (about 5,000-year half-life) are valuable for long-term 

 experiments — particularly those in intermediary metabolism. C u is useful 

 for in vivo studies since, as a positron emitter, it has penetrating annihilation 

 gamma rays. It can be prepared in carrier-free form in the cyclotron by the 

 reaction B 10 (d, n)C u . Because of excellent cyclotron yields, it can also be 

 used for making simple organic compounds for preliminary studies to be 

 followed later with more exacting studies using C 13 or C 14 . C 13 must of 

 course be measured with the mass spectrometer; C 14 , which emits weak 

 beta particles and no gamma rays, must be counted under fairly exacting 

 conditions, i.e., with a thin mica-window counter and exact geometry or, 



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