PRIMING REACTIONS 



201 



When he found that malate and oxaloacetate had the same action as 

 succinate and fumarate, and that their intervention is aboHshed by malon- 

 ate, Szent-Gyorgyi introduced the oxaloacetate-malate system into his 

 scheme, which became : 



AH, 



X 



CHaCOOH 



I 



CO.COOH 



Oxaloacetic 



acid 

 CHj COOH 



I 

 CH(OH)COOH 



Malic 

 acid 



COj.CGOH 



I 



CHXOOH 



Succinic 



acid 



CH COOH 



II 



CH COOH 

 Fumaric 

 acid 



ox. cytochrome 



red. cytochrome 



H,0 



1/2 O2 



dehydrogenase of 

 AH, 



malic 

 dehydrogenase 



succinic 

 dehydrogenase 



cytochrome 

 oxidase 



When it was demonstrated that the succinic-fumaric system has no 

 coenzyme whilst the oxaloacetic-mahc system requires the mediation of 

 DPN, the idea of the couphng of the two systems was recognized as 

 impossible and it was abandoned. 



But it was known that fumarase catalyses the malate-fumarate trans- 

 formation, and that malic dehydrogenase catalyses the oxaloacetate-malate 

 conversion. From which facts sprung a new formulation of the cycle of the 

 dicarboxylic acids. 



In the new scheme, the system catalysed by fumarase and malic dehydro- 

 genase is a supply system inserted laterally into the first scheme of Szent- 

 Gyorgyi : 



malic dehydrogenase 



"chTcooh 



•CO COOH 

 oxaloacetic acid 

 CH2COOH 

 CH(OH)COOH 

 malic acid 



DPNH 



DPN 



AH 



fumarase i:H20 



CH COOH 

 =^/ ^HCOOH Y '"""^'^ cytochromey y,o, 

 Y fumaric acid I \ 



A. CHsCOOH^^^j^.^^^ cytochrome^VHp 

 -^ ^ CH2COOH 

 succinic acid 



-V- 



"v 



dehydrogenase of 

 AH2 



succinic 

 dehydrogenase 



Cytochrome 

 oxidase 



