IV. BIOCHEMICAL SYSTEMS 431 



tion of Westonbriiik and van Dorp as by that of Lipton and Elvehjcm. 

 There was one oxi)orimont that was ox))hiinod more easily ])y the Wisconsin 

 than by the Amsterdam investigators: Lipton and Elvehjem demonstrated 

 that by incubating the apoenzyme with cocarboxylase at 30° the recovery 

 of oarboxyhi^e gradually increased as the incubation period increased. How- 

 ever, Westenbrink and his collaborators could not confirm these results 

 with the kind of yeast they had at their disposal. 



Buchman et al.^* found a competitive inhibition of thiamine pyrophos- 

 phate b}' the thiazole pyrophosphate portion of the cocarboxylase molecule. 

 Therefore they assumed that the thiamine pyrophosphate is bound to the 

 apoenzyme through the ])yrophosphate group which is common to both the 

 cocarboxylase and the inhibitor. 



We know a great many reactions that are catalyzed by thiamine-con- 

 taining enzymes; they are listed in Table I. Little is known as yet of the 

 question whether all these reactions are catalyzed by different enzymes 

 (containing different apoenzymes). 



These enzymes have not yet been obtained in a pure, crystalline state. 

 All are protein-thiamine pyrophosphate-magnesium compounds. Fairly 

 pure preparations have been made from yeast. ^^' ^' The composition of both 

 preparations did not differ very much; per mole of thiamine pyrophosphate 

 they contained 1 g. of atom of magnesium and about 75,000 g. (1 mole?) 

 of protein. 



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