DISCUSSION ON HYDROGEN TRANSPORT 35 



probably not through oxalacetate-malate, since the redox potential 

 of the latter system is close to that of pyruvate— lactate.* According 

 to the complete theory, fumarate and malate should behave alike, 

 an equilibrium mixture of the two being rapidly produced by the 

 action of fumarase when either is added. But Dr. Libet and I have 

 found that added malate and fumarate (and citrate) have different 

 effects on the repiration of brain suspensions. Greig and Munro 

 found that fumarate, but not malate, caused a lowered respiratory 

 quotient with ox retina and chick embryo. 



3. What metabolites are oxidized through the system? The Szent- 

 Gyorgyi school indicated that carbohydrate derivatives— triose- 

 phosphate, alpha-glycerophosphate, and pyruvate— were oxidized 

 through the four-carbon dicarboxylic acid system by tissue sus- 

 pensions. But Greville reported that not more than 70 per cent of 

 the respiration brought about by fumarate catalysis in muscle was 

 due to carbohydrate oxidation. With suspensions of liver from 

 fasted rats, my wife and I found low respiratory quotient values 

 for the extra respiration caused by adding malate. Leloir and Munoz 

 found that added four-carbon dicarboxylic acids increased the rate 

 of butyric acid oxidation by liver suspension. Dewan and Green, 

 with isolated enzyme preparations, showed the oxidation of beta- 

 hydroxybutyrate by fumarate. Annau, and my wife and I, noted 



* Dr. Eric Ball, Harvard University: My chief objection to the Szent- 

 Gyorgyi theory is that the inclusion of the malate-oxalacetate system in the 

 chain of reactions as it is written seems to be pointless. Attention has been called 

 to tliis fact previously ( 4-6 ) . If we break the scheme into the separate reactions, 

 this becomes evident. Assuming the substrate to be oxidized by means of 

 diphosphopyridine nucleotide (Py(P04)2), we may write the first reaction as fol- 

 lows: 



( 1 ) Substrate + Py(P04 ) 2 -> H^PyCPO^ ) 2 + Oxidized Substrate 



If the pyridine nucleotide is to act as a cyclic catalyst for this reaction, it must 

 be oxidized. According to the Szent-Gyorgyi scheme, tliis reoxidation is brought 

 about by oxalacetate. The reaction may be written: 



( 2 ) Oxalacetate -|- H2Py(P04 ) 2 -> Py(P04 ) 2 -f Malate 



Now if the oxalacetate in turn is to function as a catalyst, it must be regen- 

 erated. This requires that malate be oxidized. The oxidation of malate in the 

 body, however, is known to proceed only through the diphosphopyridine nucleo- 

 tide: 



( 3 ) M alate -f Py (PO4 ) 2 ^ H2Py(P04 ) 2 + Oxalacetate 



This equation is, however, the reverse of equation 2. Thus what is produced is 

 reduced pyridine nucleotide, and we are right where we started when we wrote 

 equation 1. The introduction of the malate-oxalacetate system into diis cycle 

 merely leads us into a blind alley. 



