FORMATION OF MILK 305 



only those inorganic phospliorus atoms which were located in the plasma 

 at some time after the start of the experiment, the specific activity 

 of the milk inorganic phosphorus should be as high as that of the plasma 

 inorganic phosphorus. In making such a comparison, it must be borne 

 in mind that the specific activity of the plasma inorganic phosphorus 

 rapidly decreases with increasing time through interaction of plasma 

 phosphate phosphorus with that of bone and other tissue. No definite 

 conclusion can therefore be drawn from comparing a single value of 

 the specific activity of plasma and milk phosphorus. By following up, 

 however, the change of the specific activity of the plasma inorganic- 

 phosphorus and milk inorganic phosphorus with time, we find that i1 

 takes 3 — 4 hours for the milk inorganic phosphorus to be almost entirely 

 composed of individual atoms which had been present in the plasma 

 after the start of the experiment. 



In milk produced shortly after the start of the experiment, a large 

 part of the phosphorus atoms present were those which were located 

 in the milk gland when the labelled phosphorus was administered. 

 The replacement of the gland inorganic phosphorus by plasma inorganic 

 phosphorus is thus comparatively slow because of a slow rate of penetra- 

 tion of the phosphate ions through the cell walls. Heavy water, on the 

 other hand, injected simultaneously with the labelled phosphate was 

 already, after a short time, equally distributed between plasma and 

 milk, because of the low resistance water molecules encounter when 

 penetrating through cell walls. 



Casein phosphorus 



The comparatively high specific activity of the casein phosphorus is 

 only compatible with the assumption that the phosphorus atoms utili- 

 zed in the synthesis of the casein in the milk gland are drawn from the 

 inorganic phosphorus of the plasma. From the difference in the rates 

 at which the active casein phosphorus and the active inorganic phos- 

 phorus present in the milk are formed, the time of formation of the 

 casein in the gland cells can be estimated to be about 1 hour. 



Ester phosphorus 



The rate of formation in the milk gland of the average labelled phos- 

 phorus ester molecule is lower than that of the average casein molecule 

 (cf. table). II/4 hours after the administration of radioactive hexose- 

 monophosphate (kindly presented to us by Prof. Parxas) injected into 

 the veins of the goat, an appreciable amount of labelled ester was found 

 in the milk, while another larger part of the activity was found in the 

 inorganic milk phosphate. This result shows that a rapid enzymatic 



20 Hevesy 



