37. NUCLEIC ACID AND PROTEIN SYNTHESIS 393 



faecalis cells an enzyme, free of amino acid activation activity and appar- 

 ently of sRNA, which will stimulate the incorporation of amino acids 

 into "washed" rat liver microsomes in the presence of ATP. It is as ef- 

 fective as the pH 5 fraction in this respect. The only other catalytic ac- 

 tivity of the preparation is an incorporation of all four C 14 -ribonucleoside 

 diphosphates into the corresponding triphosphates, an activity which 

 parallels the amino acid incorporating activity during purification. 



An enzyme preparation derived from the soluble protein fraction of 

 rat liver has been found by Sachs 191 to stimulate incorporation of amino 

 acids into liver microsomal protein. This enzyme ("S-protein") differs 

 from the pH 5 enzyme fraction, or whole supernatant fraction, in that 

 it requires glutathione for activity, and is assumed to be free of sRNA. 

 Cohn 62 has described a method for preparing microsomal particles by use 

 of the non-ionic detergent "Lubrol W." These particles are said to be capa- 

 ble of incorporating amino acids into their proteins almost as well in the 

 absence of soluble enzymes as in their presence. 



Rendi and Hultin 63 have combined the use of the "S-protein" of Sachs 

 and Lubrol W treated particles of Cohn (after a further extraction of 

 the latter with KC1 to remove indigenous activating enzymes) to obtain 

 an active incorporation system which appears to have no sRNA component. 

 (In neither the Sachs, nor the Rendi and Hultin experiments was the 

 RNA content of the soluble fraction measured, however.) The system 

 is, nevertheless, dependent on both of these enzymic components as well 

 as ATP, GTP, and glutathione. These authors conclude that incorporation 

 may occur via pathways not involving sRNA-amino acid intermediates. 



It may prove to be that there are, in fact, alternate pathways of protein 

 synthesis. Certainly the results on studies of protein synthesis in other 

 cell fractions point to this possibility. However, this conclusion is not 

 justified from the data presented in any of the above experiments. These 

 experiments may, in fact, be taken as examples of the principle that the 

 failure to demonstrate the requirement for an intermediate does not 

 establish its inessentiality. Microsomes and particles derived therefrom 

 are grossly "contaminated" with activating enzymes, and presumable 

 transfer RNA and other tissue components may also be present. Experi- 

 ments of Stephenson in our laboratory have shown that extremely small 

 quantities of sRNA (50 ng.) are required for the transfer of activated 

 amino acids to relatively large quantities of microsomal protein (8 mg.). 96 

 Each system undoubtedly has a different limiting component; if this is 

 sRNA-amino acid, as in the experiments which use it as the only source 

 for amino acid in protein, then its essentiality is apparent. If, on the other 

 hand, the limiting factor is an enzyme required, for example, in the trans- 



191 H. Sachs, J. Biol. Chem. 228, 23 (1957). 



