APPLICATIOIJS OF ISOTOPIC INDICATORS 949 



The conclusion that the formation of the phosphatide molecules of 

 the plasma mainly occurs in the liver^^^' "'*• *^^^ is strongly supported 

 by the results obtained in the study of fat phosphorylation in the hepa- 

 tectomized dog by Chaikoff and his colleagues /''^^ These authors 

 injected labelled sodium phosphate intravenally immediately after re- 

 moval of the liver. Practically no phosphatide ^^p ;vas recovered in the 

 plasma as late as 3 — 6 hours after extirpation of the liver ; at these 

 times 0.4 per cent of the injected ^^P had been incorporated into phos- 

 phatides of both kidneys and about an equal amount into the whole 

 small intestine. 



Considerations similar to those applied to the origin of the phosphorus 

 compounds of the yolk were used in an investigation of the phosphorus 

 compounds of milk.^^^^ As seen in Table 7, the milk phosphatides 

 were found to have a much higher specific activity than the plasma 

 phosphatides, indicating that the phosphatides must enter the milk 

 from a source other than the plasma and must thus have been synthe- 

 sized to a large extent in the mammary gland. The determination of 

 the specific activity of the mammary gland phosphatides revealed a 

 very high value, even higher than those found for the kidney and liver 

 phosphatide P. 



Table 7. — Specific Activity of the Phosphatide 



P Extracted from the Organs of a Goat 4^4 



HoxTBS after Administration of Labelled 



Sodium Phosphate 



Fraction Specific activity 



Milk 0.09 



Plasma 0.02 



Corpuscles j 0.01 



Mammary gland | 0.13 



Liver ' 0.09 



Kidney 0.11 



Plasma inorganic P i 1.48 



One often encounters the view that the milk fat originates from the 

 phosphatides of the blood, which are decomposed into fatty acid and 

 inorganic P in the mammary gland. The inorganic P present in the milk 

 should, according to this view, originate from phosphatide P. The fat 

 content of goat's milk amounts to about 3 per cent. Taking the ratio 

 of fatty acid to phosphorus to be 20: 1 in plasma phosphatides, the 

 production of 3 per cent fatty acid from phosphatides would set free 

 0.15 per cent of inorganic phosphorus. This being about the inorganic 

 P content of the milk, almost all inorganic P of the milk should originate 



