BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 509 



Binkley's^*^ paper ends with interesting, but highly speculative, hypothe- 

 ses on the mechanism of protein synthesis; the first stage is ascribed to 

 PNA, acting as dipeptidase for the synthesis of dipeptides. Such a sugges- 

 tion would be in keeping with Linderstr0m-Lang's^^^'®^ observation that 

 dipeptidases are always found in large amounts in active protein-synthe- 

 sizing cells, as well as with Fruton's'^^ and Chant renne's^*^ views on trans- 

 peptidation reactions in protein synthesis. 



b. Energy-rich Bonds in PNA Synthesis 



The necessity for an energy supply for peptide synthesis has been em- 

 phasized repeatedly, and Borsook^^* has conclusively demonstrated that 

 incorporation of labeled amino acids into proteins requires energy: the 

 process is stopped, or markedly reduced, by anaerobiosis or addition of 

 cyanide, azide, dinitrophenol, etc. In Siekevitz's'^^ opinion, this uptake of 

 labeled amino acids into proteins is more closely linked to phosphorylation 

 than to oxidation. 



Such a conclusion is in keeping with Lipmann's"® and Chantrenne's^^^ 

 views on the mechanism of peptide bond synthesis and with the possibility, 

 which has been successfully tested experimentally by Chantrenne,^*^ that 

 phosphorylated amino acids might be intermediaries in peptide synthesis. 



It should be pointed out, however, that these problems relate to peptide 

 synthesis only, and protein synthesis might be a very different process. 

 It should also be noted that Borsook's^^* experimental findings might 

 have an alternative explanation. It has been recently reported by Simp- 

 son^^^ that not only the uptake, but also the liberation, of amino acids 

 from liver slices is markedly inhibited by anaerobiosis, cyanide, dinitro- 

 phenol, etc.; it might thus well be that liberation of amino acids from 

 intracellular proteins also requires energy. If such were the case, proteolysis 

 might occur by two different mechanisms, one of which required energy. 

 If this were so, one of the reasons for doubting the intervention of proteases 

 in protein synthesis would lose much of its weight. 



The suggestion has been made by Spiegelman^^' that phosphorylated 

 PNA might be a specific phosphate and energy donor for protein synthesis; 

 later work by Spiegelman and Kamen^*^" has shown, however, that what 

 had been taken for phosphorylated PXA probably is a mixture of ordinary 

 PNA and metaphosphate. The whole question of the frequent association 

 of metaphosphate and PNA is, however, still in a very confused state, and 

 much more work will be required before the existence of phosphorylated 

 PNA can be accepted or rejected. 



i'8 M. V. Simpson, J. Biol. Chem. 201, 143 (1953). 



^^' S. Spiegelman, Cold Spring Harbor Symposia Quant. Biol. 11, 256 (1946). 

 ^°'' S. Spiegelman and M. D. Kamen, Cold Spring Harbor Symposia Quant. Biol. 12, 

 211 (1947). 



