STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 151 



the effect of growing mouse fibroblasts (strain L) in suspension in the presence 

 of DPN. Such cells have a considerably depressed DPNase activity, appar- 

 ently resulting from the formation of stable, inactive,enzyme-nicotinamide 

 derivative complexes. It appeared that exogenous DPN could enter these 

 cells. Other enzymes levels were not affected and, indeed, there was no 

 indication that the synthesis of DPNase was affected, since DPNase disap- 

 peared in far less than a division time. Thus, the first examination of 

 mammahan systems from this point of view has revealed a new type of 

 control of enzymatic activity in these cells. This instance may be of par- 

 ticular interest to our understanding of the origin of enzymes, such as 

 neuraminidase, which have only been found in infected cells. Does the virus 

 introduce the enzyme which, in cleaving substrate, releases inhibited neur- 

 aminidase? Or does virus infection in some way lead to the induction of the 

 synthesis of the enzyme? The very formulation of these problems in these 

 terms should facilitate their analysis. 



VI. Biochemical Mechanisms of Polymer Formation 



The clarification of mechanisms of polymer formation has barely begmi. 

 Only the most recent textbooks in biochemistry contain significant informa- 

 tion in this area. However, a number of reviews are useful in surveying recent 

 thought and experimentation with respect to problems of the synthesis of 

 protein (Borsook, 1954, 1956; Fruton, 1954), of nucleic acid (Ochoa and 

 Heppel, 1957; Kornberg, 1957a,b), of polysaccharides (Hassid, 1954; 

 Kalckar, 1954; Stacey, 1954; Edelman, 1956), and of phospholipids (Kennedy, 

 1957). 



As noted in earher sections, the formation of the complex substances 

 indicated above is dependent upon active metabolism. More particularly, the 

 formation of the peptide bonds of proteins, the phosphodiester bonds of 

 nucleic acids, the glycosidic linkages of polysaccharides, and the ester 

 linkages of the phospholipids requires the intermediary development of high- 

 energy bonds in compounds such as adenosine triphosphate (ATP) and 

 acyl-SCoA. The concept that such bonds comprise the operating currency 

 and driving substances of the energy-requiring reactions of the cellular 

 economy was developed largely by Lipmann (1941) and its material bases 

 have been growing continuously since his initial presentation of the theory. 



It should be noted that a "high-energy bond" has a different significance 

 from the concept of "bond energy" in the field of energetics. The latter 

 signifies the energy which must be introduced into a molecule to break a bond 

 between two atoms. Thus, the disruption of a P to bond in ATP may 

 require an energy input of 50 to 100 Idlocalories per mole. On the other 

 hand, the biochemist is concerned primarily with the change in chemical 



