AMINO ACID INCORPORATION BY LIVER MICROSOMES 237 



(i960)) suggest that the 70 S ribonucleoprotein particle is actively engaged in pro- 

 tein synthesis, but that the 30, 50 and 100 S particles are not. Since a 30 S and a 

 50 S particle aggregate to form the active 70 S particle, it is possible that factors 

 (such as Mg^+ concentration) which influence the aggregation and disaggregation 

 of these particles, may regulate the number of active sites available at a single mo- 

 ment for protein synthesis. 



Packer: Detergents are one clearway by which one can dissociate microsomes; 

 another clear way is the type which has just been put on the board and this type 

 has been documented quite nicely for microsomes of pea seedlings. Recently we 

 have been examining suspensions of liver microsomes by adding anions and cations 

 and chelating agents such as ATP, and following the changes in scattered light in 

 the suspensions, and we find very large changes in light scattering accompany the 

 addition of these reagents ; so I would like to suggest that these normal chemical 

 substances which are present in the cell are capable of aflfecting the state of struc- 

 ture of particles and perhaps the reaction system you employ for measuring the 

 incorporation of labelled amino acids. 



HuLTiN : Perhaps I should point out again that we always have a good deal of 

 magnesium present, and the magnesium content, as shown by Dr. Peterman, is 

 very important in determining the aggregation of the particles. I think that factor is 

 under control as we use a little more than the physiological concentration of 

 magnesium. 



Packer : I feel that this sort of approach must be considered if you are to evalu- 

 ate properly the structural-functional relationships especially as the types of 

 changes in light scattering which I have mentioned occur very rapidly. 



HuLTiN : We agree that this has something to do with structure because when 

 we treat the particles with detergents in a solution of high ionic strength we find 

 the effects fade off. 



Porter: I would like to ask what you think happens to the microsomal 

 membrane when you detach the RNP particle from the microsomes with DOC or 

 any other of these reagents. Does a fragment go with the particle ? 



HuLTiN : No. The particles we obtain in this way are absolutely free from all 

 the enzymes which are typical of the microsomes. For example, from gIucose-6- 

 phosphatase, from DPNH-cytochrome c reductase, from TPNH-cytochrome c 

 reductase, from cytochrome 56, etc. This means that they have very little impurities 

 from the membrane. 



Campbell : Have you any idea what happens to the microsome pellet when you 

 incubate it, when it loses its ability to incorporate ? 



HuLTiN : I am very interested in this point but unfortunately I can't say very 

 much about it yet. 



