IV. HIOCHEMICAL SYSTEMS 427 



the oxygen uptake was due not to thiamine itself but to a compound that 

 was synthesized from the free \itamin (Peters el al}^ and Westenbrink and 

 Pohik"). Tlie nature of this compound became clear from another series of 

 investigations. Xeuberg and his collaborators (Neuberg and Karczag'^ and 

 Xeui)erg and Rosenthal'^) found an enzyme in yeast which decarboxylates 

 pyruvic acid to acetaldehyde. They called it carboxylase. Simola'^ showed 

 that the tissues of rats on a thiamine-deficient diet had a greatly reduced 

 content of carboxylase. 



All the known thiamine-containing enzymes (holoenzymes) consist of a 

 protein part that is usually called the apoenzyme and a coenzyme of lower 

 molecular weight, the thiamine pyrophosphate (or sometimes thiamine tri- 

 or polypho-sphates). All these enzymes catalyze either a pure decarboxyla- 

 tion process or an oxidative decarboxylation reaction. 



Both take part in the metabolism of pyru^^c acid or, more generally 

 speaking, in the metabolism of a-oxycarboxylic acids. 



We distinguish between pure carboxylases and oxidative carboxylases, 

 which are usually called pyru^'ic dehydrogenases. The carboxylases are 

 present in yeast and other microorganism, whereas animal tissues contain 

 py ru vodehy drogenases . 



Whether the carboxylase and the pyruvodehydrogenase are identical or 

 are different enzymes will be discussed below. 



Auhagen'^' ^^ demonstrated that carboxylase could be rendered inactive 

 by washing with a phosphate buffer solution at pH 7 to 8. The activity was 

 restored b}' adding an extract of boiled yeast. So he assumed that yeast con- 

 tains a coenzyme, which he called cocarboxylase. Lohmann and Schuster'^ 

 succeeded in isolating this cocarboxylase in a pure, crystalline state, and 

 they proved that it is the pyrophosphoric ester of thiamine. This coenzyme 

 + alkali-washed yeast + magnesium ions decomposes pyruvic acid ac- 

 cording to the reaction: 



CH3COCOOH _£^lb25y!5i«_^ CH3CHO + CO.. 



Peters and his pupils demonstrated that the cocarboxylase is the active 

 form of thiamine in tissue oxidation, also (Banga et al}^- '^). 

 The bond between the apo- and the cocarboxylase at neutral or slightly 



•0 R. A. Peters, H. Rydin, and R. H. S. Thompson, Biochem. J. 29, 53 (1935). 



" H. G. K. Westenbrink and J. J. Polak, Rec. trav. chim. 56, 315 (1937). 



" C. Xeuberg and L. Karczag, Biochem. Z. 36, 68 (1911). 



" C. Xeuberg and P. Rosenthal, Biochem. Z. 51, 128 (1913). 



'^ P. E. Simola, Biochem. Z. 254, 229 (1932). 



'* K. .\uhagen, Z. phiisiol. Chcm. 204, 149 (1931). 



" K. .Vuhagen, Biochem. Z. 258, 3.30 (19.33). 



'^ K. Lohmann and P. Schuster, Biochem. Z. 294, 188 (1937). 



" I. Banga, S. Ochoa, and R. A. Peters, Nature 144, 74 (19.39). 



" I. Banga, S. Ochoa, and R. A. Peters, Biochem. J. 33, 1109 (1939). 



