518 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 25 



than the intact molecule that cannot be synthesized. An exception is 

 1-lysine, which is required in the intact state [N48]. A process of de- and 

 reamination of almost all acids goes on continuously in biological systems; 

 it has further been found that, when N 15 -labeled ammonia is administered to 

 organisms ranging from viruses to higher plants and animals, it becomes 

 widely distributed in the constituent proteins of the organism. In regard 

 to nitrogenous waste products, N 15 has been particularly useful in giving a 

 clear-cut answer to the source of creatine and creatinine [N10]. The sources 

 of creatine are glycine, the amidine group of arginine, and the methyl group 

 of methionine. In the mammal all creatinine is derived directly from body 

 creatine, and the reaction is irreversible in vivo [N12]. 



The use of N 15 for labeling amino acids, especially coupled with C 13 or 

 C 14 in the carbon chains, will unquestionably reveal a vast amount of as yet 

 unknown facts of protein metabolism. It is through such studies that we 

 should derive some degree of fundamental understanding of the complexities 

 of protoplasm and genetic continuity. 



25.6. Sulfur. For sulfur, a component of many proteins and certain 

 nutrilites essential for many organisms, there have been two isotopes applied 

 to biological problems: a rare stable species S 34 and an unstable species S 35 

 (87.1-day half-life). There are two other rare stable species that may con- 

 ceivably find tracer application, S 33 and S 36 . 



5 34 has had very limited application, so far having been applied biologically 

 only to the study of the conversion of labeled methionine to cystine in 

 vivo [S6], 



5 35 has been available in relatively limited quantities until recently. The 

 nuclear-pile reaction Cl 35 (n, p)S 35 is superior in yield to the several cyclotron 

 reactions. Nevertheless, S 35 has been applied to a number of studies on 

 sulfur metabolism, particularly the intermediary metabolism of sulfur- 

 containing amino acids and proteins [S21-26] and the excretion of sulfur 

 wastes as sulfate [S7,25]. It has been used to demonstrate that inorganic 

 sulfate is not reduced and incorporated into the body protein [S25] and that 

 inorganic sulfide is largely oxidized to sulfate although small amounts may 

 be incorporated into protein [S7,8,19]. Preliminary studies have also been 

 carried out with labeled thiamin, which is in a state of dynamic equilibrium 

 just as are the major organic metabolites [S3]. 



25.7. Phosphorus. Phosphorus, a constituent of bone, of many organic 

 energy-storing and -transferring systems, and of nucleoprotein, which is 

 basic to all life as we know it, has but one stable isotope, P 31 , and for tracer 

 work but one biologically useful form, the radioactive species P 32 (14.3-day 

 half-life). There are more publications dealing with this isotope than with 

 any other species, stable or radioactive. P 32 may be produced in excellent 



