D. RITTENBERC AND D. SHEMIN 



glycine to an actively immunized ral)l)il at llie stage at vvhicli the titer 

 is lalling results in the rapid incorporation of the isotopic nitrogen in 

 the antibody even in the phase of decline (32). 



The rates of protein synthesis and destruction have been studied 

 in growing tissues, regenerating ii\er, and tumor tissues. Since it 

 has been shown by the isotope technique that tissues are in a state of 

 flux, it is reasonable to conclude that the synthetic or anabolic rate in 

 a tissue of constant weight must be equal to the degradative or cata- 

 bolic rate. In a growing tissue, however, the rate must be greater 

 than the degradative rate. This disparity in rates may be the result 

 of a relative increase of the synthetic rate as compared with that in 

 the nongr owing tissue, of a relative decrease of the rate of degradation, 

 or of a combination of the two possibilities. Which of the three 

 possibilities operates and accounts for growth in regenerating liver 

 tissue can be ascertained by the isotope technique. From unpublished 

 data obtained in our laboratory, it appears that, in the regenerating 

 tissue, the synthetic rate is similar to the rate found in the nongrowing 

 liver but the degradative rate is markedly decreased. Growth is the 

 result of an inhibition rather than the initiation or acceleration of re- 

 actions. It seems likely that the next few years will see an intensive 

 study of the rate of synthesis of various tissue components under diverse 

 physiological conditions in the intact animal. 



An extension of the isotope technique is the isotope dilution 

 method of analysis (25), a technique which has been very useful in 

 various problems, particularly that of accurately determining the 

 amount of diflferent compounds in a mixture. The technique is based 

 on the fact that a compound which contains more than the normal 

 abundance of isotope is inseparable from its normal analogue by the 

 usual laboratory procedures. In this procedure, a small amount, for 

 example, of isotopic glutamic acid is added to a protein hydrolyzate 

 and a representative sample of glutamic acid is isolated from this 

 mixture. From the amount of glutamic acid added (A) and its N'^ 

 concentration (Co) and the N^^ content of the isolated glutamic acid 

 (C), the amount of glutamic acid (B) originally present in the mixture 

 can be calculated from the formula: B = A[(Co/C) - 1]. This 

 technique applied to the determination of the amount of an amino 

 acid in a pure protein has provided the most reliable analytical values 

 at present available (14,38). One of the interesting developments of 



272 



