AMINO ACIDS IN PRE- AND POST-NATAL PERIODS 265 
same is true for several amino acids in respect to leukocytes of patients with leuke- 
mias3—35. 
In addition to the “placental pump mechanism” (DENT), there are theoretically 
several other ways to satisfy the high protein demand of the fetus. Very briefly, the 
following possibilities exist®*®: 
1. Proteolytic enzymes of the placenta could split maternal proteins into amino 
acids. This is unlikely since active proteolytic enzymes have not yet been detected 
(efarets037,°38)* 
2. The placenta could synthesize proteins destined for the fetus. No reliable 
studies bearing on this point seem to have been done. 
3. Some proteins of the mother are transported intact through the placenta. Ex- 



Prot. 
Counts/min 
80 000 
70 000 
60 000 amino acids 
50 000 
40 000 
else hoes: insoluble proteins 
20 000 x oe wes proteins 
is a RiGee 
10 000 3 habekaeld « 
ig eS 
1 ae 41 —— 2 eee 
22 23 24 25 26 27 28 

Days 
Fig. 1. Specific activity of proteins and free amino acids in brain of 
rabbit fetus following intraperitoneal administration of !C-lysine. 
tensive literature exists concerning this problem (see refs. 39-42). From these studies 
one may conclude, that practically only y-globulin can pass through the placenta. 
Despite the fact that albumin has a much smaller molecular size, it permeates the 
placental membrane to less than 10%, of the values for y-globulin. 
After cutting the cord of rats and rabbits a very steep decline of the incorporation 
rate of labeled amino acids takes place (Fig. 2). 
I have not been able to find corresponding studies in the human newborn. But the 
changes of amino acids in blood and urine indicate that the reaction proceeds in the 
same way. 
In later life the regulation of amino acid levels in body fluids is achieved by: 
1. Homeostatic processes. These are governed unquestionably by the liver with its 
great capacity to concentrate and incorporate amino acids. The existence of other 
as yet poorly understood regulative processes, which are responsible for the degree 
of the reaction, is demonstrated, for instance, by the very different breakdown rates 
References p. 279/283 
