IV. BIOCHEMICAL SYSTEMS 493 



G. SPECIFICITY 

 1. DPN-TPN Specificity 



Most DPN-TPN-linked enzyme systems exhibit a definite preference, if 

 not a specific requirement, for either DPN or TPN. Well-known exceptions 

 to this are liver glutamic dehydrogenase^^' " and glucose dehydrogenase'^^, 

 which react equally well with either TPN or DPN. Mehler et al?'' studied 

 the specificity of the DPN or TPN requirement for a number of enzyme 

 systems. Malic and lactic dehydrogenases, for instance, reacted with either 

 DPN or TPN, but the reaction rates were many times faster with DPN. 

 On the other hand, isocitric dehydrogenase is strictly TPN-specific,"' ^^' ^^ 

 whereas triosephosphate dehydrogenase is strictly DPN-specific.^^ 



Earlier reports which indicated a relatively non-specific DPN-TPN re- 

 quirement for certain dehydrogenases may have been complicated by the 

 fact that certain liver" and yeast'*- preparations can catalyze the conversion 

 of DPN to TPN if ATP is present. It is also well known that some tissues 

 contain phosphatases which convert TPN to DPN.^^- ^^ 



In addition Colowick and associates'^' '^ have obtained evidence of an 

 enzyme in Pseudomonas fluorescens extracts which appears to catalyze the 

 reaction TPNH2 + DPN -> TPN -f DPNH2. An enzyme which carries 

 out this reaction has been observed in animal tissue.'^ 



2. Specificity of the Molecule 



Only a few modifications or derivatives of the DPN-TPN molecules 

 have been studied to determine the range within which these molecules 

 can be modified and still retain enzymatic activity. Desamino DPN can 

 be prepared by treating DPN with nitrous acid'^ or with an enzyme de- 

 rived from takadiastase,^^ the free amino group of the adenine moiety 



^^ H. von Euler, E. Adler,, G. Giinther, and N. B. Das, Hoppe-Seyler's Z. -physiol. 



Chem. 254, 61 (1938). 

 "A. H. Mehler, A. Kornberg, S. Grisolia, and S. Ochoa, /. Biol. Chem. 174,961 



(1948). 

 " H. J. Strecker and S. Korkes, J. Biol. Chem. 196, 769 (1952). 

 ^9 S. Ochoa, J. Biol. Chem. 174, 133 (1948). 



8» E. Adler, H. von Euler, G. Gunther, and M. Plass, Biochem. J. 33, 1028 (1939). 

 81 G. T. Cori, M. W. Slein, and C. F. Cori, /. Biol. Chem. 159, 565 (1945). 

 *2 H. von Euler, E. Adler, and T. S. Eriksen, Hoppe-Seyler's Z. physiol. Chem. 248, 



227 (1937). 

 83 N. O. Kaplan, S. P. Colowick, and M. M. Ciotti, /. Biol. Chem. 194, 579 (1952). 

 8^ S. P. Colowick, N. O. Kaplan, E. F. Neufeld, and M. M. Ciotti, /. Biol. Chem. 



195, 95 (1952). 



85 N. O. Kaplan, S. P. Colowick, and E. F. Neufeld, /. Biol. Chem. 195, 107 (1952). 



86 J. Stern, quoted in Federation Proc. 11, 238 (1952). 



8' F. Schlenk, H. Hellstrom, and H. von Euler, Ber. 71, 1471 (1938). 



