2o: 



SUBCELLULAR PARTICLES 



160 



20 



K concentration = 0.0135 m 



Molarity of NaCl 



-J I I L_ 



_] I I I I I I 



Fig. 7. Effect of varying the so- 

 dium ion concentration of the in- 

 cubation medium on the abiUty of 

 thymus to incorporate glycinc-i-C". 

 The specific activity of the nuclear 

 protein after 60 minutes' incuba- 

 tion of the nuclei in the presence 

 of glycine-C^* is plotted against the 

 sodium concentration of the me- 

 dium. 



0.02 0.04 0.06 0.08 QIO 0.12 014 



amino acids as well, including alanine, leucine, lysine, phenylalanine and trypto- 

 phan. The sodium ion requirement seems to be specific. 



Figure 8 summarizes the results of experiments in which an attempt was made 

 to substitute all or part of the sodium in the medium with an equivalent amount 

 of potassium. Only the ratio NaCl/KCl was varied; the total salt concentration 

 was kept constant. When all the sodium is replaced by potassium the uptake falls 

 to 15 per cent of the optimal value. Increasing the sodium to potassium ratio gives 

 a corresponding increase in the amount of amino acid taken up. When the sodium 

 concentration is optimal, the addition of small amounts of potassium (chloride) 

 does not influence the uptake. (Similar experiments show that lithium can par- 

 tially replace sodium in nuclear function.) 



It has already been pointed out that osmotic balance is important to nuclear 

 activity. Yet this is only a part of the over-all requirement for maintaining nu- 

 clear function. Nuclei removed from sucrose to isotonic salt solutions lose their 

 ability to incorporate amino acids. Nuclei washed with neutral o.i m phosphate 

 bufl^er prior to incubation in a sucrose medium are also inactive. The presence of 

 the sucrose seemed to be essential in maintaining nuclear structure. 



To test whether this requirement was specific, or whether other sugars could 

 replace sucrose, nuclei were isolated in a wide variety of media and then tested 

 for their capacity to incorporate radioactive amino acids. Without presenting the 

 details of such isolations, the main conclusions can be briefly summarized. It was 

 found that the nature of the sugar made little difference to the synthetic capacity 

 of the isolated nucleus. Other disaccharides (maltose, lactose) in 0.25 m solu- 

 tions permitted the isolation of nuclei which were just as active as those prepared 

 in sucrose; the monosaccharides tested (fructose, glucose) also gave preparations 



