STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



91 



In the experiments of Littlefield et al. (1955; Littlefield and Keller, 1957), 

 it was observed that, when a C^*-amino acid is injected intravenously into a 

 rat, the initial incorporation into the microsomal fraction of the liver is 

 several times greater than into other cell fractions. Furthermore, as shown in 

 Fig. 13, when this fraction was separated into the deoxycholate-insoluble 



40 



^ -30 

 o 



E 

 i- 20 



u 10 



10 15 



Time (min) 



20 



25 



Fig. 13. Incorporation in vivo of a small dose of leucine-C^'* into the two components 

 of the microsomes and into the soluble protein of the cell (Littlefield et al., 1955). The 

 per cent RNA by weight of each deoxycholate-insoluble sample is indicated. 



ribonucleoproteins and deoxycholate-soluble hpoproteins, the former 

 possessed a markedly greater activity. In addition, under the conditions of 

 the experiment presented in Fig. 13, i.e., using a small dose of labeled 

 leucine-C^^, it appeared that after initial fixation, the newly formed protein 

 left this fraction at about the rate the specific activity of the deoxycholate- 

 soluble fraction increased. That amino acid incorporation in these systems 

 involves peptide bond formation has been demonstrated by Zamecnik and 

 KelJer (1954). 



It may be asked if such an apparent localization does not arise from 

 primary peptide synthesis in the nucleolus, transport to the cytoplasm, and 

 selective fixation on the microsomal components. However, appearance of 

 isotope in the microsomal fraction is iimnediate and contiimes linearly 

 (Littlefield and KeUer, 1957). Furthermore, we shall see that enucleate cell 

 fragments are capable of protein synthesis. Finally, as will be discussed in 

 greater detail below, the isolated microsomal fraction is capable of incor- 

 porating amino acids in vitro, and a transport mechanism need not be i]ivoked 

 under these experimental conditions. 



A number of interesting variants of the results of this basic incorporation 

 experiment may be mentioned. The incorporation of methionine-S^^ into the 



