INCORPO RATIO iX OF '*C INTO TISSUE 799 



of ^^C into the fatty acid of homogenates of the marrow of irradiated 

 rabbits. 



The application of ^^C as indicator may thus reveal radiation lesions 

 both in the irradiated organ and other organs, due either to changes in 

 the turnover rate or to the change in sensitivity of the indicator, only. 



SOME ASPECTS OF ACETATE METABOLISM 



To the daily diet of the rat we can add a constant amount of acetate 

 labelled in the carboxyl group and obtain an almost contant activity 

 level of the body acetate. This activity level is measured by feeding 

 phenylbutyric acid to the rat, isolating the excreted phenylbutyric 

 acetate, and determining its ^^C content. 



By comparing at the end of the experiment, the i*C content of the 

 fatty acid carbon of the liver with the corresponding value of the urinary 

 phenylbutyric acetate, as shown by Rittenberg and Bloch,^^) the rate 

 of the liver fatty acids can be calculated. 



In a recent investigation, Pihl et. alS^^ have shown that prolonged 

 daily feeding of a constant amount of labelled acetate does not lead to a 

 constant but to an increasing activity level of the body acetate with 

 time. This contrasts with the results obtained w^hen feeding heavy 

 water which results in a practically constant deuterium level of the body 

 fluids. Though intake of heavy water is followed by the formation of 

 deuteriated body constituents which, in turn, are catabolized and form 

 deuteriated water, in view of the large water content of the body (about 

 70 per cent) these additional amounts of catabolic heavy water do not 

 much influence the deuterium content of the body water. In a similar 

 way, the catabollsm of ^'^C labelled fatty acids and other compounds 

 ensues in the formation of labelled acetate. In view of the low (much 

 less than 1 per cent) acetate content of the body fluids the additional 

 catabolic labelled acetate can not, how^ever, be disregarded and corre- 

 spondingly, in experiments of many days, through which a constant 

 amount of labelled acetate is fed, a quite marked increase in the activity 

 level of body fluid acetate takes place. 



A very different behaviour of the labelled acetate is observed when it 

 is administered, for example by intraperitoneal injection, at the start 

 of the experiment(^). The catabolism of the administered acetate leads 

 to a large extent to the formation of ^^COg, which is exhaled. In the rat 



(i^D. J. RiTTENBERG and K. Bloch, Biol. Chem. 160, 417 (1945). 

 (2) A. Pihl, K. Bloch and H. S. Anker, J. Biol. Chem. 183, 441 (1950). 

 ^3) cf. G. Hevesy, R. Ruyssen and M. L.Beeckmans, Experientia 7, 144 

 (1951). 



