Sec. 25.3] MAJOR ORGANIC METABOLITES 515 



in the case of low activities, in a gas counter or ionization chamber. It has 

 a definite advantage over C 13 in providing a greater sensitivity of measure- 

 ment. It can be prepared in high specific activity in the nuclear pile by the 

 reaction N 14 (n, p)C 14 , but in practical operation a considerable amount of 

 stable carbon contaminates this process. 



The most significant work with C 11 has been studies on carbon dioxide 

 utilization by animals and plants [C29, 125, 169, 174] and on carbon monoxide 

 metabolism [C147]. In the latter case it has been shown that carbon 

 monoxide is not oxidized, at least over a 2-hr period in the human body, into 

 carbon dioxide. 



With C 13 a rather large series of experiments have been performed dealing 

 with the intermediary metabolism of carbohydrates, fats, and, to a lesser 

 degree, amino acids. A recent review by Vennesland [CI 56] gives an 

 excellent account up through 1947. Much information has been derived on 

 the role of organic precursors. Among the most important results has been 

 discarding of the idea that certain simple substances are the specific precursors 

 of more complex compounds, e.g., lactic acid as a precursor of glycogen. 

 Rather, they are now regarded as contributors to the general metabolic 

 pool from which the complex substances are formed. In connection with 

 carbon dioxide metabolism, a number of studies have established a revolu- 

 tionary fact, namely, that carbon dioxide is not merely an end product of 

 organic metabolism in the higher animals, but actually an essential com- 

 pound in certain organic metabolic reactions, although admittedly these do 

 not compare in complexity with bacterial chemo- and photosynthesis or 

 plant photosynthesis. 



C 14 , having only recently become available in appreciable quantities from 

 the nuclear-pile reactor, is just beginning to have widespread application. 

 It seems destined to become the isotope of choice as a tracer in most future 

 research in intermediary metabolism of proteins, carbohydrates, fats, 

 vitamins, organic pharmaceuticals, etc. In fact, it shows promise of becom- 

 ing the most important single radioisotope available for tracer work. The 

 most important work with C 14 reported up to the end of 1947 is probably the 

 demonstration that radioactively labeled carbonate may be laid down in 

 rat bone for at least several weeks without showing any change in concentra- 

 tion; this suggests great caution in the application of this isotope to humans 

 [C24]. Other important studies have been on the distribution in the rat of 

 C 14 -labeled amino acids [C69,173, etc]. (Since this volume was submitted 

 to press, an important monograph by Calvin et at. [C37a] on the properties 

 and use of the carbon isotopes has appeared.) 



25.3. Hydrogen. Hydrogen, which with carbon is found in all organic 

 compounds and which with oxygen comprises, in the form of water, the 

 greater part of living organisms, has two useful tracer isotopes: the rare 



