114 THE BIOSYNTHESIS OF PROTEINS 



On the other hand, indirect and circumstantial evidence for the existence 

 of peptide intermediates has been presented on several occasions. Peptides 

 obtained by the action of proteolytic enzymes on proteins stimulate protein 

 synthesis more than a mixture of amino acids in certain systems (Kihara 

 and Snell, 1955; Rychlik and Sorm, 1956; Fox and Krampitz, 1956). The 

 rather frequent occurrence of individual tripeptides in certain classes of 

 proteins (Sorm, 1957, 1959, 1960) makes one wonder whether these tri- 

 peptides might not be made separately and serve as precursors for several 

 related proteins. So far, however, all the studies on the utilization of 

 labelled di- or tripeptides have shown that the amino acids are used more 

 readily than the peptides and that these must actually be split into amino 

 acids to be utilized. The amino acids are the real precursors. This was 

 found for instance in the case of the synthesis of haemoglobin in reticulo- 

 cytes (Nizet and Lambert, 1954), and of casein in the goat (Godin and 

 Work, 1956), for amino acid incorporation in tissue slices (Hendler and 

 Greenberg, 1954) and for bacterial growth (Meinhart and Simmonds, 

 1955). The stimulatory effect of peptides obtained by enzymic hydrolysis 

 of proteins is not completely explained. It is due in certain cases to glutam- 

 ine and asparagine (Rychlik and Sorm, 1957), which are contained in 

 enzymic hydrolysates of proteins, but not in acid hydrolysates since these 

 amides are split by acid into ammonia and glutamic or aspartic acid re- 

 spectively. It is indeed established at present that glutamine and asparagine 

 are incorporated as such into proteins (Barry, 1954, 1956; Rabinovitz et ah, 

 1956; Sansom and Barry, 1958). They should be regarded as individual 

 amino acids which are just as different from glutamic and aspartic acids 

 as e.g. leucine or glycine, as far as incorporation into protein is concerned. 

 The 'complete' amino acid mixtures used in the overwhelming majority of 

 the experiments did not contain any asparagine or glutamine, and were 

 therefore incomplete mixtures of protein precursors. 



Another possibility is that peptides might in certain cases be degraded 

 by living cells in such a way that they would provide activated amino acids 

 directly (Walter et ah, 1956). This also deserves further investigation. 



A more striking phenomenon which might reflect the existence of 

 peptide intermediates of some sort is the non-uniform labelling of proteins 

 as observed most clearly with in vitro systems. Pieces of hen oviduct in- 

 corporate the carbon of ^'^002 in vitro into ovalbumin and other proteins. 

 Ovalbumin which had been labelled in this manner was split by a bacterial 

 protease into two fragments, according to Ottesen and Wollenberger (1953). 

 The larger fragment, named plakalbumin, can be isolated in a crystalline 

 form, the smaller one is a hexapeptide. The specific radioactivity of the 

 aspartic acid residues of these two fragments differed by a factor greater 

 than four (Steinberg and Anfinsen, 1952). A similar result was later ob- 

 tained for the incorporation of labelled phenylalanine and glycine into 



