BIOCHEMICAL PROPKRTIKS OF THE ISOLATED NUCLEUS 



201 



centration is presented in figure 6. There is a fairly sharp optimum at a sucrose 

 concentration near isotonic (i. e. at 0.20 m in a medium which also contains sodium 

 phosphate buffer (0.025 m), glucose (0.02 m) and NaCl (0.03 m)). Although the 

 effects described bear some resemblance to the osmotic behavior of semipermeable 

 membranes, it should be pointed out that isolated thymus nuclei are not enveloped 

 by an intact, semipermeable membrane and that they arc freely permeable to 

 large molecules such as ribonuclease (mol. wt. 15000), deoxyribonuclease (mol. 

 \vt. 60000), nucleic acids, histones, protamines, polylysine and basic dyes. It is a 

 surprising aspect of nuclear organization that it is freely permeable to molecules 

 of this size yet is able to retain its mononucleotides (such at ATP) and its histones 

 even when the DNA is removed (6). 



The marked effects of sucrose concentration on nuclear activity suggest that the 

 fine structure necessary for nuclear syntheses is in osmotic balance with its en- 

 vironment and that nuclear function can vary with that balance. This conclusion 

 finds support in parallel observations that the sucrose optimum for orotic acid- 

 6-C^' incorporation into RNA (0.13 m) is far below the concentration optimum 

 for protein synthesis in the same nuclei (0.20 m). 



One of the most striking effects on nuclear amino acid incorporation is that 

 produced by sodium ions. Nuclei isolated in sucrose by the procedure described, 

 need a sodium supplement in order to incorporate amino acids actively. Figure 7 

 shows the effect of varied sodiuin concentration on the level of glycine- i-C^* up- 

 take. The sodium requirement has been found to exist for the uptake of other 



120 



Fig. 6. Effect of varying the su- 

 crose concentration of the medium 

 upon the abihty of thymus nuclei 

 to incorporate alanine- i-C". The 

 specific activity of the nuclear pro- 

 tein after 60 minutes' incubation is 

 plotted against the sucrose concen- 

 tration of the incubation medium. 



10 20 30 40 50 



SUCROSE CONCENTRATION 



(MOLES PER LITER) 



