D. RITTENBERC AND D. SMEMIN 



carbon skeleton of one coiiipoinul loi' (he foiination of another. Tvahel- 

 int;- a compomul willi carbon i,sol()|)es is (liiis the most direct nielhod 

 for demonstratinej conversions. 



The biochemical isoiopc (cchniqnc has been applied in four 

 fields: (1) in delectinL; die conxcrsion of one compound to another; 

 (2) in studying llie mechanism of biochemical reactions; (3) in measur- 

 ing the rates of reactions; and (4) in determining the amount of a 

 constituent in a mixture. It has the advantage over all other methods 

 that it may be carried out on the intact organism under normal physio- 

 logical conditions. 

 "^ The first biological experiment in which the isotope technique 



( was used was worked out by Hevesy in 1923 (15). He traced the 

 transport of lead in plants whose roots were immersed in a medium 

 containing a radioactive isotope of lead. This type of experimentation 

 marked time until Urey, in 1933, discovered and concentrated heavy 

 water. At about the same time, artificial radioactivity was discovered. 

 As a result, there became available to the biochemist concentrates 

 either of heavy isotopes or of radioactive isotopes of all the elements of 

 importance in the study of intermediary metabolism. 



Beside the numerous individual problems which in the past 

 ten years have been attacked by this method, the isotope technique 

 has revealed one phenomenon of fundamental significance in the living 

 cell, viz., the dynamic state of tissue constituents. These experiments 

 have shown that practically every component of the animal body is 

 constantly degraded and resynthesized at a rapid rate, and that the 

 former distinctions between structural and metabolic components of 

 the cell or of endogenous and exogenous metabolism are nonexistent. 

 The maintenance of the form and structure of the adult cell or tissue 

 is the result, not of a static state in which no reactions take place, but 

 rather of a dynamic equilibrium in which the synthetic and degradative 

 reactions proceed at equal rates. The existence of this stationary 

 state has been shown by experiments with several isotopes and with 

 every type of tissue, with the possible exception of the tissues of the adult 

 brain. Not only is the form and structure of the organ governed by 

 the rates of synthesis and degradation of its constituents but also its 

 composition. While the autolysis of tissues which becomes apparent 

 on the death of an animal was formerly thought to involve reactions 

 which began after death, we now believe that many of these degrada- 



268 



