440 THIAMINE 



bound to some other yeast protein. Contrary to the supposition of Sperber,** 

 Westenbrink et al}^ proved that the synthesis of carboxylase by living yeast 

 cells is not a simple reversion of the action of the phosphatase, the enzyme 

 that decomposes the cocarboxylase. 



As we take into account the tremendous activity of the newly discovered 

 cobalamin (vitamin B12), we must not exclude the possibility that the car- 

 boxylase preparations of animal cells contain a small quantity of a power- 

 ful oxidizing coenzyme other than thiamine pyrophosphate. 



At the International Biochemical Congress in Paris in July, 1952, there 

 was held a symposium on the tricarboxylic acid cycle (the citric acid cycle) .^^ 

 Most of the participants in this meeting considered it established that the 

 oxidation of pyruvic acid is performed by a thiamine pyrophosphate-con- 

 taining protein with other coenzyme(s) as the oxidizing agent(s). The oxi- 

 dizing agent might be the protogen^^ or a-lipoic acid that was isolated by 

 Reed et al.^^ in crystalline form. Green^^ states that the pyruvic oxidase 

 and other a-keto fatty acid oxidases require at least four prosthetic groups 

 to catalyze the oxidative decarboxylation of their substrates: (1) thiamine 

 pyrophosphate, (2) magnesium ions, (3) pyridine nucleotide, and (4) coen- 

 zyme A. 



So the decarboxylation and the oxidation of pyruvic acid are separate 

 steps in the oxidation of pyruvate. Only the first step, the decarboxylation, 

 is catalyzed by the thiamine pyrophosphate as a coenzyme. The product 

 of decarboxylation seems to be not free acetaldehyde but an enzyme- or 

 coenzyme-bound acetaldehyde derivative which, upon transfer of the acet- 

 aldehyde-group to coenzyme A (see pantothenic acid page) undergoes 

 oxidation as an acetaldehyde-CoA compound, (cf. Ochoa, p. 81 in ref. 84). 



Langenbeck^^ made model experiments on enzyme reactions. Referring to 

 these experiments, Weil-Malherbe^' thinks that a Schiff base is first formed 

 and that this base undergoes an intramolecular oxidation-reduction process. 

 No experimental evidence was given for this view. On the contrary Stern 

 and Melnick^" presented experimental data against the Langenbeck cycle. 



Lipmann'^ reduced thiamine with platinum black and H2 or by Na2S204. 

 He assumed that a dehydro derivative might be formed which forms a redox 



83 E. Sperber, Naturwissenschaften 29, 765 (1941). 



*^ Symposium on the tricarboxylic acid cycle held at the Second International Con- 

 gress of Biochemistry, Paris, 1952. 

 85 D. J. O'Kane and I. C. Gunzalus, J. Bacterial. 56, 499 (1948), 

 8« L. J. Reed, Science 114, 93 (1951). 

 " D. E. Green, Science 115, 661 (1952). 



88 W. Langenbeck, Ergeb. Enzymforsch. 2, 314 (1933). 



89 H. Weil -Mai her be, Nature 145, 106 (1940). 



^0 K. G. Stern and J. L. Melnick, /. Biol. Chein. 131, -WT (1939). 

 "F. Lipmann, Nature 138, 1097 (1936). 



