306 ADVENTURES IN RADIOISOTOPE RESEARCH 



breakdown of the hexosemonophosphate and rebuilding of ester mole- 

 cules takes place in the gland. The milk gland contains thus enzymes 

 having the same action on hexosemonophosphate as Robison and 

 Kray's^ bone extracts; however, the bulk of the esters present in the 

 milk are acted on by enzymes present in the gland at a much slower 

 rate. Similar behaviour is shown by the mixture of phosphorus esters 

 present in the bloocP. 



Phosphatide phosphorus 



The formation of active phosphatide molecules is, as seen from the 

 table, a slow process. The individual phosphatide molecules present in 

 the milk were mainly built up in the milk gland and not taken up as 

 such from the plasma (as is the case with the yolk phosphatide). This 

 follows from the fact that the specific activity of the phosphatide phos- 

 phorus extracted from the milk gland and also from the milk itself 

 is higher than that secured from the phosphatide of the plasma. The 

 view is often encountered that the milk fat originates from the plasma 

 phosphatides which decompose in the milk gland, supplying fat and 

 inorganic phosphorus. This view is entirely incompatible with the results 

 obtained by us. To mention only one argument, we find the phosphatide 

 phosphorus of the milk to be slightly, the inorganic phosphorus present 

 to be strongly, active. The latter can therefore only originate from the 

 highly active inorganic phosphorus of the plasma. 



It is well known that different milk fractions, secured consecutivel}' 

 within a short time, have a markedly different fat content. As we find^ 

 that the inorganic phosphorus extracted from these fractions has a 

 different specific activity, we have to conclude that these fractions 

 cannot originate from an initially homogeneous liquid. So we arrive 

 at the result that some of the milk gland cells give off milk much more 

 readily than others, but that some even of the first-mentioned cells 

 retain a large part of their solid milk constituents, particularly the 

 phosphatides (and fats). Not only are phosphorus compounds present 

 in the milk not formed during the act of milking, as often assumed, 

 but such compounds contained in the last fraction secured during the 

 act of milking are partly of earlier date than those present in the im- 

 mediately preceding milk samples. 



References 



L. Hahn and G. Hevesy Nature 140, 1059 (1937). 



R. RoBisoN The Significance of Phosphoric Esters in Metabolism (New York, 

 1932). 



^ A detailed account of the experimental results obtained will be found in the dis- 

 sertation of A. W. Aten, jun., to be presented to the University of Utrecht. 



