504 J. BRACKET 



into proteins, and even synthesis of a specific enzyme protein, are funda- 

 mentally cytoplasmic processes. 



It is interesting to note that very similar conclusions have been drawn 

 by Bonner'^^- in studies on the genetic control of enzyme formation in 

 Neurospora: the facts that enzyme formation requires the presence of free 

 amino acids, that the process rapidly leads to the formation of a specific 

 enzyme, and that intermediates do not seem to accumulate, all point in 

 the same direction: enzyme formation probably occurs in a cytoplasmic 

 particle, and it is "the ability of such particles to replicate in an active 

 form that is genetically controlled." 



If we recall now that cytoplasmic PNA. which is certainly associated 

 with protein synthesis, is mostly accumulated in the microsomes and that, 

 in Amoeba, these microsomes disappear after removal of the nucleus, it is 

 tempting to identify them with Bonner's^®^ hypothetical cytoplasmic 

 particles. It is therefore to be expected that microsomes play some funda- 

 mental role in protein synthesis. 



(4) The Role of the Microsomes in Protein Synthesis. In their first papers 

 on the chemical composition of cytoplasmic particles, Brachet and Jeener"^ 

 (see also Brachet^^O pointed out that there is no reason to believe that 

 PNA alone plays a part in protein synthesis: it is quite possible that the 

 whole granule, the microsome, is the active agent. 



They found some support for this hypothesis in the fact that particles 

 obtained in the ultracentrifuge (which were in fact mixtures of mitochon- 

 dria and microsomes) always contained an appreciable amount of the 

 specific protein synthesized by each organ: this was found for trypsin and 

 insulin in the pancreas, amylase in salivary glands, hemoglobin in red 

 blood cells, and the melanophore-expanding hormone in the pituitary. 

 More recently, McShan and Meyer^^^ found more than 50% gonatropin in 

 cytoplasmic granules. It should be added, however, that in none of these 

 experiments was a separation made between microsomal and mitochondrial 

 fractions. 



More direct evidence comes from work done in other laboratories with 

 labeled amino acids. Borsook et aL'** were the first to report that incor- 

 poration in liver tissue is highest in the microsomes after intravenous 

 injection of labeled amino acids (glycine, lysine, and leucine). The same 

 result was also obtained by Hultin^^^ using glycine-N^* in the chick: the 

 uptake in vivo of the amino acid by the microsomal protein took place 



'" D. M. Bonner, in "Phosphorus Metabolism" (McElroy and Glass, eds.), Vol. 2, 

 p. 153. Johns Hopkins Press, Baltimore, 1952. 



•" W. H. McShan and R. K. Meyer, Proc. Soc. Exptl. Biol. Med. 71, 407 (1949). 



'" H. Borsook, C. L. Deasy, A. J. Haagen-Smit, G. Keighley, and P. H. Lowy, Fed- 

 eration Proc. 9, 154 (1950). 



'«5 T. Hultin, Exptl. Cell Research 1, 376, 599 (1950). 



