356 



ADVEXTURES IN RADIOISOTOPE RESEARCH 



90- day-old rat (3200 mgm%) is % times that of the 4-day-old rat (2800 

 mgm%) and the weight of the liver increases simultaneously from 0.24 gm 

 to 4.0 gm (Lang 1937). The phosphatide content of the 90-day-old rat 

 is thus 20 times that of the 4-day-old rat. The percentage turnover of 

 the 4-day-old rats being 1.5 times that of the 90-clay-old animals, the 

 amount turned over in the course of 1 hour in the 90-day-old animals is 



540 



10 20 30 40 50 60 70 80 90 



Age in days 



540 



Fig. 1. Percentage incorporation of orthophosphate P resp. ATP P^.z 

 per hour into liver phosphatides of rats of different age 



15 times that renewed in the 4-day-old rats, the figures being 10.25 mgm- 

 and 0.69 mgm, respectively. If between the phosphatide and the ortho- 

 phosphate (or ATP) molecule a phosphorus compound were interposed 

 which was formed at a comparatively slow rate, it would be the ultimate 

 precursor of the phosphatide molecule ; then the above mentioned figure 

 would represent the lower limit of the amount of phosphatides turned 

 over only. 



Spleen Phosphatides 



Since we do not know the mean value of the orthophosphate P of 

 the spleen during the experiment we cannot calculate the lower limit 

 of turnover rate. Assuming the mean value to be 2/3 of the end vahic 



