516 2. ANALOGS OF ENZYME REACTION COMPONENTS 



thiamine-PP is involved in metabolic reactions leading to ATP, so that 

 interference with thiamine-PP formation or function will tend to deplete 

 the cells of ATP and perhaps further depress thiamine-PP synthesis. 



The possible sites of action for thiamine analogs can be broadly classified 

 as (1) inhibition of thiamine-PP synthesis, either on the formation of thia- 

 mine or its pyrophosphorylation, (2) interference with the formation of 

 complexes between thiamine-PP and enzymes, and (3) inhibition of thia- 

 minase. In any case the inhibition may be exerted by either the analog or 

 its phosphorylated derivatives. There is no evidence that any significant 

 effects of any of the analogs studied can be attributed to thiaminase inhi- 

 bition, so the first two mechanisms are undoubtedly the most important 

 in the induction of thiamine deficiency symptoms. There is some evidence, 

 which will be discussed later, that thiamine may have a function or func- 

 tions unassociated with coenzyme activity, particularly in the nervous sys- 

 tem, and, if this is true, one might consider the interference by analogs 

 in this function. 



It would appear that most of the groups in the thiamine molecule parti- 

 cipate in either the binding or the catalysis inasmuch as the structure can 

 not be significantly altered without loss of activity, and the number of ef- 

 fective analogs is rather small. The first report of enzyme inhibition by a 

 thiamine analog was by Buchman et al. (1940), who found yeast pyruvate 

 decarboxylase activity to be depressed by 4-methyl-5-hydroxyethylthiazole 

 diphosphate (which they called "thiazole pyrophosphate"), the phosphory- 

 lated thiazole portion of thiamine. Neither the nonphosphorylated com- 

 pound nor the monophosphate is inhibitory. It requires about 10 times as 

 much analog as thiamine-PP to inhibit 50%, but if the analog is added 

 before the thiamine-PP, the inhibition is more pronounced. These results 

 point to the importance of the phosphate groups in the binding. They 

 state, "We conclude that there has been demonstrated here a not hitherto 

 recognized type of competitive inhibition of enzyme reactions, caused by 

 competition not between substrate and inhibitor but between coenzyme 

 and inhibitor." (See formulas on page 517). 



Either the pyrimidine portion or the thiazole portion of the thiamine 

 molecule can be altered to form analogs. Replacement of the thiazole ring 

 with a similarly substituted pyridine ring gives pyrithiamine, which was 

 shown by Robbins (1941) to inhibit the growth of certain fungi, and by 

 Woolley and White (1943 b) to produce thiamine deficiency symptoms in 

 mice. Replacement of the pyrimidine amino group with a hydroxyl group 

 leads to oxy thiamine, found by Bergel and Todd (1937) to lack vitamin 

 activity, and by Soodak and Cerecedo (1944) to be quite toxic to mice. 

 These two analogs have been studied the most thoroughly of the thiamine- 

 like compounds and remain the most frequently used to produce experi- 



