14 A SYMPOSIUM ON RESPIRATORY ENZYMES 



oxalacetic acid. These hypothetical reactions are shown in Figure 5. 

 In this or in a similar manner, by the way of a phosphomalic acid, 

 as Lipmann (31) and Hastings (28) recently suggested, the gap 

 between pyruvic acid and enol-phosphopyruvic may be bridged with 

 simultaneous oxidation of four-carbon acids. Indeed, with the clos- 

 ing of this gap the whole chain of reactions leading from lactic acid 

 to glycogen will be completely understood. Furthermore, the real 

 point of attack of insulin, which has so far eluded all investigators, 

 may even be sought in the oxidative mechanism concerned with the 

 four-carbon and five-carbon acids, lying on the pathway of oxidative 

 sugar breakdown. Such an assumption, already proposed by Krebs 

 (32), has not been conclusively proved by experiments. 



Other questions concerning the intermediate metabolism of carbo- 

 hydrate remain unsettled. One of these would seem to be relatively 

 easy to attack with our present facilities. Fermentation is inhibited 

 by not too high concentrations of cyanide, nitric oxide, hydrogen 

 sulfide, and o-phenanthroline, substances known to fonn complexes 

 with heavy metals. Such complex formation is responsible, as we 

 know from the work of Warburg and many others, for the inhibition 

 of respiration by these and similar substances. We cannot be sure 

 at present whether this explanation holds good also for fermentation 

 and glycolysis, since we do not know of any heavy metal indis- 

 pensable to fermentation. Lohmann (33) of Heidelberg discovered 

 that magnesium was essential for the phosphorylating enzyme sys- 

 tem and for the carboxylase (34, 34a), and magnesium can be re- 

 placed in many instances by still smaller concentrations of man- 

 ganese (35). But it is doubtful whether the latter is the metal re- 

 sponsible for the inhibitions, and we do not know which of the many 

 intermediary reactions these inhibitors attack. On the other hand, we 

 know that fluoride attacks mainly the enolase, which dehydrates the 

 2-phosphoglyceric acid to enol-phosphopyruvic acid, while oxalate 

 inhibits the dephosphorylation of the latter (36). But here too the 

 mechanisms are unknown. Only in the case of iodoacetic acid, which 

 affects the oxidoreduction steps in which cozymase takes part, does 

 the mechanism of inhibition seem to be explained, namely, by the 

 oxidation of the sulfhydryl groups of the dehydrogenase proteins 

 (37). 



Therefore our pride in the progress achieved in the last decades 

 must be tempered by confession of ignorance regarding many cru- 

 cial points. There are still many problems for this generation of 

 research workers to solve. 



