524 NIACIN 



many of the details of this mechanism. The door to this fruitful chapter of 

 biochemistry was opened by the report of Krehl et al?° that tryptophan 

 could substitute for nicotinic acid in counteracting the unfavorable growth 

 effects of corn in certain rat diets. Shortly thereafter it was shown that 

 tryptophan markedly increased the urinary excretion of N^-methylnicotina- 

 mide,^^'^^ the first clear indication that tryptophan could substitute for 

 nicotinic acid by virtue of the fact that it was transformed into nicotinic 

 acid. Subsequent reports have fully confirmed the fact that tryptophan 

 can substitute for nicotinic acid in the diet of rats, dogs, pigs, rabbits, chicks, 

 ducks, monkeys, and cotton rats (p. 578). Impressive but indirect evidence 

 that tryptophan is converted into nicotinic acid has been obtained in 

 man,^^-*^ rats,^^- 26. se. 4o-48 dogs,^^ mice,^" rabbits," horses and cotton rats,^^ 

 pjgg^36, 52 calves, sheep, and guinea pigs.^^ Most of these studies have shown 

 increased urinary excretion of nicotinic acid and its derivatives, or increased 

 tissue levels following tryptophan administration. 



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