307 



Comment on papers 27—33 



In 1935, after mailing paper 16 to Nature we embarked on the study whether 

 and to what extent the constituents of the brain are renewed during adult life. 

 After the administration of ='-P to rats an appreciable incorporation of the tracer 

 into brain phosphatides was observed after the lapse of 1 lir or more (paper 27). 

 These results were pubhshed simultaneously with those of Artom et al. (1937) 

 and of Chaikoff et al. (1937) who demonstrated the incorporation of ^sp ji^to 

 the phosphatides present in a great number of organs. We subsequently concen- 

 trated our interest on the origin of phosphatides. Among the most fascinating 

 apphcations of isotopic tracers ranges the study of the origin of body constituents. 

 With LuNDSGAARD wc fed dogs with oil containing labelled sodium phos- 

 phate in order to find out whether an appreciable part of the increased lecithin 

 content in the blood is built up in the intestine was labelled (paper 28). These 

 experiments showed that intestinal mucosa is not the chief place of sjTithesis 

 of plasma phosphatides. It was the results of perfusion experiments (paper 29) 

 which first indicated that the hver releases labelled phosphatides to the circula- 

 tion. In other experiments (paper 30) not the removal of the labelled phospha- 

 tides- from the hver but the uptake of these from the circulation by the liver was 

 followed. These were the first experiments in which blood containing in vivo syn- 

 thetized labelled compounds was transfused. They led to the result that not only 

 is the rate of turnover of phosphatides in the liver very high, but the exchange 

 of phosphatide molecules between the hver cells and the plasma takes place at 

 a much higher rate than the corresponding process between other organs and the 

 circulation. The hver was found to be the main source of formation of plasma 

 phosphatides. This was most spectacularly demonstrated by Chaikoff and his 

 group (1946) who in the course of their very extensive and important studies on 

 phosphatide metabohsm have shown that in a hepatectomized dog, after administ- 

 ration of labelled phosphate, the formation of labelled plasma phosphatides prac- 

 tically ceases. In animals almost devoid of the higher unsaturated acids there is no 

 diminution in the phosphatide turnover in the liver. An enhanced turnover rate is 

 observed in the muscles of fat -starved rats (Hevesy and Smedley-Maclean, 

 1940). 



That the chick builds up its own phosphatides and does not avail itself of the 

 phosphatides in the yolk could be concluded from the following observation. 

 After injecting labelled phosphate into the fertilized egg, the phosphatides extrac- 

 ted from various tissues of the chick were strongly radioactive while the jolk 

 phosphatides remained inactive (paper 32). 



That the phosphatide molecules of the milk in contrast to those of the yolk 

 do not originate in the blood plasma but the former are built up in the milk gland 

 could easily be proved (paper 33). The specific activity of the milk phosphatide 

 phosphorus was found to amount to almost four times the specific activity of the 

 plasma phosphatide phosphorus, but was lower than the corresponding value 

 of the milk gland phosphatide phosphorus. The experiments were carried out at 

 a time following administration of labelled sodium phosphate to the hen in which 

 the activity of the plasma phosphatides was still increasing. In such experiments 

 a milk phosphatide phosphorus specific activity which is higher than the corres- 

 ponding value of the plasma phosphatides, excludes the plasma phosphatides 



20* 



