The Significance of Respiratory Chain Oxidations 627 



Fig. 1 are based mainly upon Slater's 'Scheme C (1958b, p. 187), with the 

 introduction of cytochrome b into the DPNH-cytochromc c mitochondrial 

 pathway in order to give a possible basis for the Antimycin sensitivity of this 

 route. 



Figure 1 shows also the recently described microsomal pathway of DPNH 

 oxidation by means of a diaphorase (fpg) which has been highly purified from 

 liver microsomes by Strittmatter and Velick (1956a, b; 1957a, b) and is 

 completely specific for the microsomal cytochrome b^, reacting with DPNH. 



The longer-known TPNH-cy tochrome c reductases of yeast (Haas, Harrer, 

 and Hogness, 1942) and liver (Horecker, 1950), and the iron-containing 

 flavoprotein, DPNH-cytochrome c reductase, prepared from heart muscle 

 (Mahler and Elowe, 1954) are respectively represented by fpj and fp4 in Fig. 1. 



A number of other flavoproteins also link v/ith the respiratory chain, 

 among them the acyl CoA-dehydrogenases described from D. E. Green's 

 laboratory, which are highly important in fatty acid oxidation. 



In fact, as Krebs and Kornberg (1957, p. 219) have emphasized, by means 

 of these reactions nearly all the major metabolic sources of energy are 

 channelled into a very few mechanisms, by transferring their hydrogen atoms 

 (or electrons) to form reduced pyridine nucleotides or flavoproteins, which 

 then yield up their energy, by means of the respiratory chain oxidative 

 phosphorylations, in the form of adenosine triphosphate (ATP) required by 

 the cell for its metabolic purposes. 



There are thus three main types of compound — DPNH, TPNH and 

 reduced flavoprotein — with which the respiratory chain has to deal. In what 

 follows we will consider some of the factors which may affect the relative 

 contribution of these groups to the metabolic pathways taken in animal cells. 



POSSIBLE ROLES OF PYRIDINE NUCLEOTIDES IN 

 DETERMINING METABOLIC PATHWAYS 



Distribution of Pyridine Nucleotides in the Cell 



Before any role of nucleotides can be discussed, it is necessary to know how 

 much total diphosphopyridine nucleotide (DPN) and triphosphopyridine 

 nucleotide (TPN) is present in cells; how much is in the oxidized and 

 reduced forms; and what is the intracellular distribution. Until quite 

 recently there was little such information available, but specific and sensitive 

 methods of analysis, due to Clock and McLean (1955) in our laboratory and 

 to Jacobson and Kaplan (1957) in Baltimore, have now partly filled this gap. 



Table 1 shows some collected data for liver cells and subceUular fractions, 

 obtained by both groups of workers. The agreement is on the whole satis- 

 factory by these quite diflerent methods. The values of Clock and McLean 

 in Table 1 show several interesting points. The most obvious is the pre- 

 ponderance of the oxidized form of DPN over DPNH in all fractions of the 

 liver. Quite the reverse applies to TPN which is present to a very large extent 



