58 II. BIOSYNTHESIS 



ties of phospholipid-P^2 extracted from fetal livers and from the placentae 

 of pregnant rats, rabbits, and guinea pigs, two to six hours after the ad- 

 ministration of Na2HP04 labeled with P*-, was higher in all cases than it 

 was in the maternal plasma. It is reasoned that, if the fetal phospholipids 

 had been derived from the performed maternal substances by placental 

 transmission only, the specific activity of the fetal phospholipids should 

 not exceed that of the maternal plasma phospholipids. These data can 

 only be interpreted to mean that the labeled phospholipids present in the 

 fetal liver were synthesized in situ. They are also in harmony with the 

 report of Gortner^^* that the highest phospholipid content is to be found 

 in the very young pig fetus. 



On the other hand, the proportion of newly synthesized phospholipids 

 extracted from tissues other than liver and placenta was less than in the 

 corresponding maternal tissues. This may be related to the poor penetra- 

 tion of the labeled Na2HP04. When labeled phosphate was injected 

 directly into the uterus, all the tissues contained newly synthesized phos- 

 pholipids, while the maternal blood contained no labeled phospholipids. 

 The rate of phospholipid turnover is higher in the fetal liver and placenta 

 than in the maternal liver, which is the most active site of phospholipid 

 synthesis in the mother. The rate of turnover is slower in the other 

 fetal tissues, but it is equal to that in the maternal liver. 



h. Egg. It has long been recognized that the rate of turnover of phos- 

 pholipid is much increased in the laying bird. Thus, Entenman et al.'^^'' 

 found that the rate of synthesis of phospholipids in the blood and oviduct 

 was markedly augmented in the hen parallel with egg production. Phos- 

 pholipid-P^2 was shown to be deposited in the egg yolk at an early period 

 after the feeding of isotopic phosphorus. ^^^ Hevesy and Hahn^^^ are of 

 the opinion that the bulk of the yolk phospholipids are obtained from 

 the plasma, which in turn obtains them from their primary site of syn- 

 thesis, namely from the liver. However, the rate of synthesis of "free" 

 lecithin and cephalin in egg yolk is the same as that of the "combined" 

 phospholipids in the vitellin fraction. ^^^ 



i. Bone. Claassen, Vogels and Wostmann^^^ demonstrated that the 

 femur and tibia of the white rat are able to synthesize phospholipids. 

 This was proved by the fact that the specific activity due to P^^ was 

 higher in the bone marrow than it was in the blood plasma. Moreover, 



2" W. A. Gortner, /. Biol. Chem., 159, 135-143 (1945). 



^^ G. Hevesy and L. Hahn, Kgl. Danske Videnskab. Selskab, Biol. Medd., 14, No. 2, 

 1-39 (1938). 



2«6E. Chargaff, /. Biol. Chem., U2, 505-512 (1942). 



2" V. Claassen, R. J. Vogels, and B. Wostmann, Nature, 169, 191-192 (1952). 



