354 HICIIAHI) SCIIWKKT AM) JOHN HISIIOP 



iiit'iils for peptide IjoikI synthesis. 'I'lie use of ('"-laheled amino acids and 

 in vitro (l)Ut not cell-free) systems is ivviewed and e\-idence that amino 

 acid iiicorpoi'ation did repivsent pi'oteiii synthesis is gi\'en in detail. This 

 prt)l)lem appeared ai^ain when cell-fi-ee systems were de\-eloped. A lai'^e 

 part of the review is devoted to analogies between protein synthesis and 

 the biosynthesis of glutaniine and glutathione or transpcptidation reac- 

 tions, since these systems were well studied at the time. Tiic problem 

 of exchange versus dc novo synthesis also was unresolved at that time as 

 was the status of free jK'ptide intermediates. Both of these problems were 

 raised in part by the "une(iual labeling" results of Anfinsen and Steinberg 

 (19olK Some of these jioints have been clai'ifi('(l; others seem to have 

 lost their significance for the time being. 



Contributions which turned out to be significant for future de^"elo|)- 

 ments in jirotein synthesis were rei^orted just about this time. In 

 Borsook's laboratoiy, the importance of RNA in protein synthesis was 

 re-emphasized by Holloway and Ripley (1952), and microsomes were 

 sliown to be the most rapidly labeled cell fraction (Borsook cf al., 1950). 

 Similar results were reported l)y Hultin (1950). Although a number of 

 experiments on the incorporation of C^'*-amino acids into protein using 

 houiogenates were reported earlier, the first experiment which clearly 

 implicated microsomes was reported at this time. Siekevitz (1952), in 

 Zam(>cnik's laboratory, reported that a mixture of mitochondria and 

 microsomes from I'at li\-er incorporated C"-alanine at a rate ec^ual to 

 that of the whole homogcnate. The role of mitochondria was recognized 

 as providing energy, and the highest amount of incorporation was into 

 the microsomal protein. These findings set the stage for the development 

 of cell-free systems which provided most of the advances in the under- 

 standing of protein synthesis in the next ten years. 



An assessment of the most significant advances from 1952 through 

 1961 is more difficult, and reflects undoubtedly some bias on the part of 

 the writers. However, since the details of tlicse studies and particularly 

 the more recent ones will form the subject matter of this review, and 

 some choices must be made to keej) the length within reasonable bounds, 

 a sunniiaiy of these advances will be attempted. The work of Siekevitz 

 (1952) led directly to the system of Zamecnik and Keller (1954), where 

 microsomes plus supernatant fraction plus an ATP-generating system 

 gave good amino acid incorporation. Studies of the function of the super- 

 natant fraction began with the discoveiy of amino acid-activating 

 enzymes by Hoagland (1955) and by Berg (1956). These enzymes could 

 i)e assayed in a numb(>r of ways, but their physiological role appeared 

 to involve the formation of amino acyl-RNA compounds (Hoagland 

 ct (iL. 1957; Ogata and Nohara, 1957). The existence of amino acyl-RNA 

 compounds was indicated by studies in other laboratories at this same 



