VITAMIN AND GROWTH FACTOR RESEARCH 



enzyme is in some way inhibited by presence of other substances, 

 or (4) because pantothenic acid, in contrast to pantetheine, is not 

 readily absorbed by this organism? All of these are possibiUties, 

 and any one of them would suffice to permit the observed result. 

 The growth experiment by itself does not favor one explanation 

 over the other. 



Without implying preference for any particular explanation 

 in this instance, it may be useful to cite evidence showing that 

 insufficient attention has been paid in the recent past to cell 

 permeability as an explanation for differing activities of closely 

 related growth factors. Current evidence (cf. 6,7) indicates that 

 conversion of pantothenic acid (or pantoic acid) to coenzyme 

 A occurs via the following pathway : 



-2H 



Pantoic 

 acid 



Pantothenic- 

 acid 



Pantothenyl- 

 cysteine 



Pantetheine < Pantethine 



+ 2H 



j8- Alanine Cysteine -2H 



+ 2H 



CO5 



ATP 



Pantothenyl 

 cystine 



^ . « Pantetheine- 

 Coenzyme A < A > U U * 



^ ATP 4 -phosphate 



Acetobacter suboxydans does not grow in the absence of one of 

 the substances shown along the main line of this conversion. 

 Pantoic acid and pantothenic acid have equal activity but are 

 much less active than pantothenylcysteine, pantetheine, 4'- 

 phosphopantetheine, or reduced coenzyme A. This could be 

 interpreted as an inability to carry out reaction b readily. How- 

 ever, further investigation showed that the activity of pantothenic 

 acid (and pantoic acid), but not of pantetheine, was greatly in- 

 creased by lowering the pYi of the medium; this increased 

 activity approximated the increased concentration of undis- 

 sociated pantothenic acid that resulted from the drop in pH., and 

 the undissociated acids very nearly equalled coenzyme A in 



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