518 2. ANALOGS OF ENZYME REACTION COMPONENTS 



mental disturbances in thiamine function.* Two other types of analog per- 

 haps deserve more attention than they have received: the imidazole analog 

 (the thiazole ring replaced with a similarly substituted imidazole ring) exerts 

 an antivitamin effect on bacterial growth (Erlenmeyer et al., 1948), and the 

 2' -n-butylpyrimidine analog produces thiamine-deficiency states in rats (Em- 

 erson and South wick, 1945), both substances being roughly of the same 

 potency as pyrithiamine and oxythiamine. A large number of interesting 

 analogs, such as those synthesized by Livermore and Sealock (1947) and 

 by Mano and Tanaka (1960), have not yet been adequately examined. 



Effects on Enzymes Dependent on Thiamine-PP 



It is clear that neither oxythiamine nor pyrithiamine is inhibitory to 

 pyruvate decarboxylase, but that the diphosphate esters can interfere with 

 the binding of thiamine-PP to the apoenzyme. Thus the yeast decarboxylase 

 is inhibited by oxy thiamine-PP but not by oxythiamine (Eusebi and Ce- 

 recedo, 1950; Navazio et al., 1956), and wheat germ decarboxylase behaves 

 similarly (Eich and Cerecedo, 1954, 1955). Pyrithiamine is noninhibitory 

 whereas pyrithiamine-PP is as effective as oxythiamine-PP (Woolley, 1951; 

 Eich and Cerecedo, 1954). Oxythiamine-PPP is inhibitory (Velluz and Her- 

 bain, 1951; Navazio et al., 1956) but it is possible that some of the action 

 results from the splitting off of a phosphate to form oxythiamine-PP, just 

 as the cocarboxylase activity of thiamine-PPP has been found to depend 

 on hydrolysis to thiamine-PP (Kiessling, 1956). It is not certain if oxy- 

 thiamine-P is inhibitory and contradictory results have been obtained. 

 These data indicate the importance of the phosphate groups for the bind- 

 ing, and this is supported by the observation of Wiethoff et al. (1957) that 

 pyrophosphate inhibits wheat germ decarboxylase competitively with thia- 

 mine-PP (the pyrophosphate must be added before the thiamine-PP). 



Thiamine-PP and the phosphorylated analogs are bound fairly tightly to 

 the apoenzyme {K„^ for thiamine-PP is usually near 0.001-0.003 mM) and 

 thus the order of addition of coenzyme and analog is important, especially 

 as the analogs are bound about 1.5-3.0 kcal/mole less tightly (Woolley, 

 1951; Stewart, 1957). If thiamine-PP is added 30 min before oxythiamine- 

 PP there is no inhibition, if they are added simultaneously there is slight 

 inhibition, but if oxythiamine-PP is added 30 min before the coenzyme 

 appreciable inhibition may be exerted (Eich and Cerecedo, 1954). Although 



* It was found later that the material originally designated as pyrithiamine was a 

 mixture (Wilson and Harris, 1949). The pyridine analog was synthesized in pure 

 form and named "neopyri thiamine" but since the active compound in the early 

 preparation was this substance, it has been generally agreed that the original name be 

 restored. The general results of the early work are not invalidated, but the true po- 

 tency of pyrithiamine is greater than indicated there. 



