514 NIACIN 



2. Tissue Distribution and Turnover Rate 



The uptake of radioactive nicotinic acid by various tissues and organs 

 was determined and the concentration followed at intervals for 15 days. 

 Uptake was highest in kidneys and lowest in erythrocytes. No radioac- 

 tivity was found in plasma after 24 hours. The excretion half-time (i.e., 

 turnover rate) was about 4 days in liver, kidney, and spleen, 5 days in 

 cardiac muscle and erythrocytes, and 8 days in brain, sternum, and skele- 

 tal muscle. Since nicotinic acid exists in tissues almost entirely as coen- 

 zymes, it seems clear that coenzymes are broken down at an unexpectedly 

 rapid rate. It is also of significance that the mouse handled nicotinic acid 

 and nicotinamide identically in the above studies. 



TABLE IX 



Relative Per Cent Distribution of Radioactivity among Urinary Metabolites 



OF C"-Nicotinic Acid and C"-Nicotinamide 12 to 24 Hours after Injection 



" Unidentified but possibly 2-pyridone of N'-methylnicotinamide. 



The rather large excretion of carbon dioxide derived from nicotinic acid, 

 as observed in the above experiments, also explains a fact noted by many 

 investigators namely, that less nicotinic acid (including all known deriva- 

 tives) may be excreted than is consumed or administered (in some species). 



3. Urinary Metabolites 



The urinary excretion pattern following administration of nicotinic acid 

 and nicotinamide was generally similar, but it differed in some details. 

 There was, also, considerable variation among species. Table IX summarizes 

 the results of Leifer et al.^^^ in four species. Paper chromatographic meth- 

 ods were used to separate the various metabolites. Trigonelline contained 

 no C^*, indicating that it is not derived from nicotinic acid, in confirmation 

 of many other workers. 



Perlzweig and associates^^^ have studied the urinary excretion pattern 



196 W. A. Perlzweig, F. Rosen, and P. B. Pearson, J. Nutrition 40, 453 (1950). 



