158 



MICROSOMAL PARTICLES 



tion of 80 S particles into 60 S and 40 S units. In pH 7.5, 0.05 u, buffer, there 

 is 50 per cent dissociation, and in pW 8.5 buffer, there is over 80 per cent dis- 

 sociation (fig. 2a, b, c). Substantially, all the subunits in pH 7.5, 0.05 u, phos- 

 phate buffer can be returned to the form of 80 S particles either by addition of 

 magnesium (5 X 10" 4 M) or by titration of the pH back to pH 6.5 (fig. 2d, e) . 

 Thus, the dissociation of 80 S particles to 60 S and 40 S units is a reversible 

 one. Increase in pH or concentration of phosphate buffer promotes the disso- 

 ciation by taking magnesium away from the particles. This interpretation is 



PH: 



(b) 



7.5 



(C) 

 8.5 



(d) 



(e) 



Fig. 2. Ultracentrifuge patterns of particles in 0.05 fi K-phosphate buffer as affected by 

 pH. Speed 35,600 rpm. (a) In pH 6.5 buffer at 0° C for 13 hours. (/>) In pH 7.5 buffer 

 at 0° C for 16 hours. (<:) In pH 8.5 buffer at 0° C for l'/ 2 hours, (d) Solution in pH 7.5, 

 0° C for 2 hours, back-titrated with phosphoric acid to pH 6.5, stored at 0° C for 3 hours. 

 (e) Solution in pH 7.5, 0° C for V/z hours. MgCl 2 added to final concentration of 

 5 X 10~ 4 M, stored at 0°C for 3 hours. 



