368 MAHLON B. HOAGLAND 



C 14 -amino acids. It may be that the cell limiting membrane (in bacteria) 

 and the membrane component of the endoplasmic reticulum (in animal 

 tissues) are necessary for the completion of the synthetic sequence by re- 

 moving the finished proteins from their site of synthesis. By breaking up the 

 cell we may disturb this relationship. This view is supported by the work of 

 Hendler 101 which suggests that lipids may be essential in protein synthesis, 

 of Hokin and Hokin 102 which suggests that transport of protein through 

 endoplasmic membrane may be an energy-requiring process, and of 

 Sachs 103, 104 whose results could be interpreted to mean that newly synthe- 

 sized protein in ribosomes fails to pass on into the soluble phase when frac- 

 tionated particles are used. 



The incubation of various cell-free tissue preparations with C 14 -amino 

 acids has led in a few instances to some accumulation of labeled nonmicro- 

 somal protein which has acquired its label later than the microsomal pro- 

 tein in analogy with events in whole cell preparations. 105 These studies do 

 not of course establish that the labeled soluble protein was derived from 

 the ribosomes. Simkin (quoted in Askonas et al. 106 ) has shown, however, 

 that if one isolates microsomes from an incubation in which their protein 

 has been labeled in the presence of ATP, soluble enzymes, and C 14 -amino 

 acid, these microsomes will release some labeled protein upon subsequent 

 incubation with unlabeled cell sap, Mg ++ , and ATP. Such an occurrence 

 might represent a release of newly synthesized soluble protein but much 

 further work is needed to establish this. 



We may well ask then whether there is any evidence that truly cell-free 

 preparations containing ribonucleoprotein particles are capable of an un- 

 ambiguous net synthesis of protein. Encouraging results along these lines 

 have been reported. 



Reticulocytes are useful as a protein synthetic system because a large 

 fraction of their synthetic product is in single protein which is relatively 

 easy to isolate and purify. Rabinovitz and Olson had shown 88 that in whole 

 reticulocytes incubated with C 14 -amino acid there was a rapid rise of radio- 

 activity in ribonucleoprotein particles, followed shortly by an equally pre- 

 cipitous fall and appearance of radioactivity in soluble hemoglobin. These 

 authors, subsequently, reported briefly 107 that this apparent transfer of 

 labeled hemoglobin from the particulate site of synthesis to the soluble 



101 R. Hendler, Science 128, 143 (1958). 



102 L. E. Hokin and M. R. Hokin, Federation Proc. 18, 978 (1959). 



103 H. Sachs, J. Biol. Chem. 233, 643 (1958). 



104 H. Sachs, J. Biol. Chem. 233, 650 (1958). 



105 J. L. Simkin and T. S. Work, Biochem. J. 67, 617 (1957). 



106 B. A. Askonas, J. L. Simkin, and T. S. Work, Proc. 4th Intern. Congr. Biochem., 

 Vienna, 1958 p. 181. Pergamon Press, London (1960). 



107 M. Rabinovitz and M. E. Olson, Federation Proc. 16, 1011 (1957). 



