ESMOND E. SNELL 



growth activity (7). Thus it appears that cells of A. suboxydans 

 are relatively impermeable to the anions of pantoic and panto- 

 thenic acids, but not to the undissociated acids. Pantothenyl- 

 cystine and pantethine are without activity, but the reduced 

 compounds, pantothenylcysteine (formula XII) and pantetheine, 

 show full activity. The presence of the — SH group permits 

 ready absorption despite presence of an ionized carboxyl group, 

 for the activity of pantothenylcysteine is not j&H-dependent as 

 is that of pantothenic acid, pantoic acid, or pantothenylglycine 

 (7). This indicates that the — SH group is concerned in 

 absorption of these compounds through the cell membrane. 



Are similar phenomena of importance in animal nutrition? 

 We do not know, but fragmentary evidence suggests that they 

 may be. Thus the conversion of pantothenic acid to coenzyme 

 A appears at present to occur in animals by the same pathway 

 shown above, and cell-free extracts of rat liver convert panto- 

 thenylcysteine to pantetheine (22). Yet, whereas pantothenic 

 acid and pantetheine have equal activities in promoting growth 

 of rats, pantothenylcysteine has none (79) . The result is difficult 

 to explain except by assuming either that the biosynthetic route 

 is incorrect or that pantothenylcysteine does not get into the 

 cells. Again, the observation that relatively massive doses of 

 vitamin B12 given by mouth are required in the absence of in- 

 trinsic factor to produce remissions in pernicious anemia, whereas 

 very small amounts are required by injection, may indicate 

 difficulty in absorption of this substance through the intestinal 

 wall. Does the intrinsic factor, by combining with vitamin 

 B12, provide a "handle" by means of which receptors in the 

 intestinal wall can transfer this vitamin to the circulation, much 

 as a free — SH group in pantothenylcysteine permits efficient 

 absorption of this substance by A. suboxydans despite the delete- 

 rious effects of an ionized carboxyl group? 



BIOGYTIN 



When yeast extract is assayed for biotin with Lactobacillus 

 arabinosus, the result is much lower than that obtained with L. 



100 



