l6 INTERMEDIARY METABOLISM AND GROWTH 



l)CHO 



CHOH 



„„ r^r^. ■*■ Glyceraldehyde-P- ^ ^ r^ors lj 



+ H,P04+DPN ^-— — ►O^C-OPOjHs 



3 dehydrogenase ■ 



CH2 0P03Hj 

 Giyceraldehyde -pnospnoie 



CHOH + DPNH +H 



I 



CHjOPOaHz 

 1, 3-Diphosphoqlycerate 



2) COOH COOH 



! I 



C=0 t DPNH + h"" '-"'^'"^ m^ CHOH+DPN'' 



Y *-" ^ uriNn T n dehydrogenase i 



CH, CH3 



Pyruvic 



Lactic 



+ Ethonol ^, , ,-s, , 



3) CHO tDPNH+H* dehydrogenase ^ ^^2°^ 



CH3 CH3 tDPN + 



Acetoldehyde Ethonol 



Fig. 6. Dismutation reactions involving DPN^. 



2. Reoxidation of reduced pyridine nucleotides via the cytochrome system 



It will be noted that in the "dismutation" system described above, pyruvate, acet- 

 aldehyde, or "pyruvate + CO2" function as the hydrogen and electron acceptors 

 from DPNH2. In most cells, however, the ultimate hydrogen and electron acceptor 

 is oxygen, and the final product formed is water. The hydrogen and the electrons 

 are not transferred directly from the substrate and DPNH-, to oxygen. Instead, the 

 transfer is mediated by a series of auxiliary carriers, which consist of flavoproteins 

 and several iron-porphyrin enzymes, the cytochromes . The iron moieties of the cy- 

 tochromes are reduced from the ferric to the ferrous state by electrons derived 

 from the substrates and are reoxidized from the ferrous to the ferric state by 

 oxygen. The cytochrome pigments have highly characteristic absorption spectra 

 which may be used for the purposes of identification (Chance, 1953-1954)- The 

 wave lengths at which maximum absorption occurs are shown in Table 3, which 

 also shows the standard electrode potentials of the components of the electron 

 transport chain and of certain other metabolites which are of interest in connec- 

 tion with electron transport. 



Two points are worthy of note in connection with the electrode potentials of Table 3. 

 First, the standard oxidation-reduction potential of riboflavin is approximately — 0.185 V 

 while that of xanthine oxidase flavoprotein is — 0.350, that of the liver mitochondrial 

 flavoproteins, approximately — 0.060, and that of the DPNH-cyt. c reductase is about 0.00. 

 Thus, the oxidation-reduction potential of a substance is greatly modified by combination 

 with proteins. Similar considerations apply with respect to the oxidation-reduction 

 potentials of the iron-porphyrin pigments. 



Secondly, the actual oxidizing power of a given couplet depends on the ratio of the 

 concentrations of the components of the couplet within the cell. Thus, the standard electrode 

 potentials of Table 3 do not reflect the actual oxidizing or reducing power of the substances 

 within the cell. The ratio of the oxidized to reduced forms of the components of the electron 



