1 70 IMMUNO-CATALYSIS 



3.5 mg. of malic acid. Similar effects were obtained in experiments 

 on the reversible dehydrogenation of succinic acid to fumaric acid. 

 Oxalacetic acid inhibited both the dehydrogenation of succinic acid 

 and the hydrogenation of fumaric acid. Malonic acid inhibited the 

 dehydrogenation of lactic, malic and succinic acids and the hydrogen- 

 ation of their respective reaction products. 



Das emphasized the point that the enzyme has greater affinity for 

 the oxidized forms (i.e., p)n:uvic acid and oxalacetic acid) than for 

 their reduced forms (i.e., lactic and malic acids). This was evident 

 from the comparatively strong inhibition of the dehydrogenation of 

 the reduced forms by pyruvic and oxalacetic acids. 



10. Inhibition of Carboxylase by Acetaldehyde, the De- 

 carboxylation Product of Pyruvic Acid 



Since the discovery of carboxylase by Neuberg it has been variously 

 reported that acetaldehyde resulting from the decarboxylation of 

 pyruvic acid inhibits the activity of carboxylase. Lohmann and 

 Schuster (1937) in their study on the isolation and properties of 

 cocarboxylase determined the inhibitory effect of acetaldehyde in a 

 system containing a yeast suspension, washed with alkaline phosphate, 

 purified cocarboxylase and magnesium. During a 30 minute reaction 

 period at optimal pH of 6.2 to 6.4 the inhibition of the decarboxylation 

 of pyruvic acid by 2, 4 and 8 mg. acetaldehyde was respectively 25 to 

 30, 50 to 55 and 64 to 67 per cent. 



11. Inhibition of the Oxidation of Purines by the Re- 

 action Products of Purine Structure 



There exists in milk and in most living tissues an enzyme capable 

 of oxidizing the purine bases hypoxan thine and xanthine. Dixon and 

 Thurlow (1924) studied this system in some detail. They used a 

 preparation from milk. They reported that uric acid, a purine base, 

 inhibited the oxidation of both hypoxanthine and xanthine, and the 

 oxidation of hypoxanthine to xanthine. Other structurally related 

 purine bases, such as guanine, adenine and xanthine itself were found 

 to produce this inhibitory effect upon the oxidation of hypoxanthine. 



