TS'O 



159 



further substantiated by experiments on dialysis of the particles. When a par- 

 ticle preparation is dialyzed against a buffer in which it is stable, dissociation 

 to 60 S and 40 S units occurs (fig. 5a, d). This dissociation is caused by re- 

 moval by dialysis of essential cofactor(s), since it can be prevented or reversed 

 by dialyzing the particles against the deproteinized supernatant (fig. 3>b, e). 

 The cofactor is present in the supernatant of the extract from which the par- 

 ticles were originally isolated; it is completely heat stable and not absorbed on 

 charcoal; finally, its protective and dissociation-reversing properties have been 

 duplicated by magnesium chloride, 5xl0" 4 M (fig. 3c, /). Calcium chloride 



Fig. 3. Ultracentrifuge patterns of particles after dialysis. Speed 36,500 rpm. (a) Dialy- 

 sis against 0.025 fi K-P0 4 , pH 6.5, 2-4° C, for 13 hours, (b) Supernatant, plus 0.025 fi 

 K-P0 4 , pH 65, 2-4° C, for 14 hours, (c) 0.025 /x K-P0 4 , pH 6.5, 5 X 10" 4 M MgCl 2 , 

 2-4° C, for 19 hours, (d) 0.025 ^ K-P0 4 , pH 6.5, 2-4° C, for 24 hours, (e) Solution first 

 dialyzed in phosphate buffer to produce solution of (a), and the solution of (a) redialyzed 

 against supernatant with 0.025 p K-P0 4 , pH 6.5, 2-4° C, 14 hours. (/) Solution first 

 dialyzed in phosphate buffer to produce a solution similar to (a), and the solution of 75, 

 60, and 40 S particles redialyzed against 0.025 p. K-P0 4 , pH 6.5, with 5 X 10' 4 M MgCl.,, 

 2-4° C, for 14 hours. 



