METABOLISM 



tinaxnide actually inhibited the growth of rats when added at a 

 level of I % to a diet containing lo % of casein. ^^ The effect was 

 probably due to a deprivation of methyl groups, for the administration 

 of methionine or of choline together with homocystine prevented the 

 inhibition of growth by nicotinamide. Choline, betaine, homocystine 

 or cystine alone had no effect. Presumably the increased severity of 

 the symptoms caused by nicotinamide is due to the greater facility 

 with which nicotinamide is methylated as compared with the acid. 

 Interference with growth on administration of nicotinamide was 

 only obserA^ed with rats ; rabbits and guinea-pigs continued to grow 

 normally, and there was no increase in the excretion of N^-methyl- 

 nicotinamide.^^ 



P. Ellinger ^^ confirmed these results, and showed that kidney and 

 muscle did not methylate nicotinamide, the enzyme being present 

 only in liver. Moreover, the enzyme was specific for nicotinamide, 

 and did not methylate nicotinic acid. Liver tissue from rats with a 

 low N^-methylnicotinamide elimination was only able to convert small 

 amounts of nicotinamide into N^-methylnicotinamide ; the addition 

 of methionine increased the amount formed. The only tissues capable 

 of converting the acid into the amide were kidney and brain ; the 

 specificity of the amidase was not tested, but the same tissues con- 

 verted glutamic acid into glutamine. When glutamine was added to 

 liver, N^-methylnicotinamide was formed from nicotinic acid. 



Other Metabolites 



The presence of another fluorescent substance in urine was reported 

 by Y. Raoul.^® Its constitution is unknown, but it was stated to be 

 an oxidation product of indole-3-acetic acid. It may be related to 

 the pigment, urorosein, the presence of which in the urine of pellagrins 

 has already been noted (page 244). Another substance apparently 

 connected with the metabolism of nicotinic acid is " pseudo-nicotinic 

 acid ", formed from trigonelline by the action of alkali ; ^® this has 

 similar properties to pyridine-3-acetic acid, with which it may be 

 identical. It is likely that " pseudo-nicotinic acid " is an artefact, 

 however, rather than a true metabolite. 



Another metabolite of nicotinic acid was isolated from urine by 

 W. E. Knox and W. L. Grossman, ^^ and identified as Ni-methyl-6- 

 pyridone-3-carboxylamide : 



o/V 



CH3 



255 



