IV. BIOCHEMICAL SYSTEMS 497 



Thus, during catabolic phases of cellular activity when organic substrates 

 are being oxidized to provide energy, a part of the energy may be stored 

 in the form of ATP. During anabolism, the process can be reversed, the 

 net effect of which is to convert organic acids to aldehydes, liberating a 

 reactive phosphate group which can be used to form glycosidic, ester, and 

 perhaps other (peptide) bonds. This mechanism could be used, for instance, 

 to supph^ the energy necessary to form the acetal bond by which glucose 

 is polymerized into glycogen. Reactions such as these represent a general 

 biological mechanism by which cells can convert the latent energy of 

 organic substances into readily stored and utilized energy. 



In addition to the type of reaction listed above, it is known that there 

 are aerobic processes which convert inorganic phosphate into energy-rich 

 pyrophosphates by reactions in which hydrogen atoms of DPNH2 and 

 TPNH2 are transported to oxygen via the riboflavin and cytochrome en- 

 zymes.^"^ Using a particulate (mitochondrial) fraction from rat liver, Fried- 

 kin and Lehninger^"^ and Lehninger'^^ have been able to demonstrate the 

 synthesis of esterified phosphate during oxidations which are DPN linked. 

 Adenosinediphosphate appeared to be the phosphate acceptor in their sys- 

 tem. Lehninger"" has recently published a careful analysis of the evidence 

 pertaining to oxidative phosphorylation in DPN-linked systems. Although 

 some of the evidence is conflicting, and many details are as yet unclear, it 

 does seem probable that phosphorylation coupled to electron transport 

 between DPNH2 and oxygen constitutes a general biological mechanism 

 for the esterification of high-energy phosphate. Green and associates,"^ 

 using a difl"erent system derived from pig heart, have also obtained evidence 

 indicating oxidative phosphorylation in DPN-TPN-linked systems. 



DPN and TPN participate in phosphorylation reactions in yet another 

 way. It has long been known that inorganic orthophosphate accumulates 

 during the respiration of certain tissues. As inorganic phosphate accumu- 

 lates, the balance between hexose phosphate and glycogen is disturbed, 

 resulting in the breakdown of glycogen to glucose-1-phosphate. This sup- 

 plies fuel for the glycolysis cycle, the energy from which can be utilized to 

 resynthesize phosphoric acid anhydrides from inorganic phosphates. As the 

 concentration of inorganic phosphate decreases and that of the organic 

 phosphate increases, the equilibrium shifts toward the formation of glyco- 

 ls F. Lipmann, Currents in Biochemical Research, pp. 137-148. Interscience Pub- 

 lishers, New York, 1946. 

 108 M. Friedkin and A. L. Lehninger, /. Biol. Chem. 178, 611 (1949). 

 i»9 A. L. Lelminger, J. Biol. Chem. 178, 625 (1949). 

 "" A. L. Lehninger, in Phosphorus Metabolism, p. 344. The Johns Hopkins Press, 



Baltimore, 1951. 

 I'l D. E. Green and H. Beinert, in Phosphorus Metabolism, p. 330. The Johns Hop- 

 kins Press, Baltimore, 1951. 



