The Significance of Respiratory Chain Oxidations 633 



the other hand TPNH, the predominant form of the second coenzyme, is 

 available in the cell, even while DPN-linked oxidations are proceeding, for a 

 series of reductive syntheses which are mainly TPNH specific. They include 

 a number of vitally important metabolic reactions (cf. Horecker and Hiatt, 

 1958a, b; Dickens et al., 1959): 



(a) Reductive carboxylations, e.g. of pyruvate to malate — an apparently 

 essential step in the net resynthesis of carbohydrate from pyruvate via the 

 'malic' enzyme, malic dehydrogenase and the reaction of Utter and Kurahashi 

 (1954), leading from oxoloacetate to phosphoewo/pyruvate. 



(b) Reductive amination of ketoglutarate to glutamate via glutamic 

 dehydrogenase. By transaminase action on glutamate, many other amino 

 acids are synthesized. 



(c) Biological hydroxylation reactions of steroids, aromatic compounds, 

 etc., of the general type: 



TPNH + H+ + O2 + phenylalanine -> TPN+ + tyrosine + HgO 



(Kaufman, 1958). 



(d) Reductive synthesis of long chain fatty acids from carbohydrates via 

 acetyl-CoA and reversal of the degradation of fatty acids, certain reductive 

 steps of which (e.g. of crotonyl-CoA reduction to butyryl-CoA) have been 

 found to be TPNH specific by Langdon (1957) and by Seubert, Greull and 

 Lynen (1957). 



(e) Reduction of folic acid (and derivatives) to tetrahydrofolic acid, a co- 

 enzyme of vital importance in one-carbon transfer anabolic reactions. 



(f) Ring closure hydroxylation and demethylation in the biosynthesis of 

 cholesterol and steroids, by the action of the oxidocyclase system, which 

 preferentially utilizes TPNH and oxygen (Tchen and Bloch, 1957a, b). 



In addition to a number of examples of such TPNH-linked syntheses 

 reviewed earlier (Dickens et al., 1959), striking examples of control by TPNH 

 in intermediary liver metabolism of cholesterol, fatty acid and protein 

 synthesis have been presented recently by Siperstein and Fagan (1958a, b) 

 and Wilson and Siperstein (1959). 



DEPENDENCE OF THE OXIDATIVE PENTOSE PHOSPHATE 

 PATHWAY ON THE LEVEL OF OXIDIZED TPN 



It is probable that the somewhat inefficient oxidation of TPNH by the 

 cytochrome system in cells is frequently a limiting factor in the operation of 

 the hexose monophosphate (HMP)-shunt in those tissues where adequate 

 dehydrogenases for glucose 6-phosphate and 6-phosphogluconate are present. 

 In such tissues, addition of substances such as oxidation-reduction dyes, 

 capable of catalytically reoxidizing TPNH, can cause a marked alteration of 

 metabohc pathway from Embden-Meyerhof to the HMP-shunt. On the 

 other hand, stimulation of processes, such as the reductive synthesis of fatty 



T 



