FUNCTIONS OF PYRIDOXAL 103 



moieties which are related to pyridoxin in structure. Detailed 

 studies of the cell-free enzymes have now proved that both 

 enzymes have the same prosthetic group, and this can be 

 substituted in vitro by pyridoxal phosphate. The actual 

 identity of the natural prosthetic group with pyridoxal 

 phosphate has yet to be established, although there is little 

 doubt but that they are the same. 



Some at least of the growth factors thus seem to function 

 as prosthetic groups of essential enzyme systems. The 

 synthetic disability may refer to the whole prosthetic group, 

 as in the case of riboflavin for S. haemolyticus, or to -a part of 

 the essential molecule as in the cases of nicotinic acid for Pr. 

 vulgaris and of thiamin for Staph, aureus. It is highly probable 

 that other growth factors are also needed by the organism as 

 parts of essential prosthetic groups. The nature of the 

 essential metabolism involved can be shown by the deficient 

 culture technique, and then, later, studies of the cell-free 

 enzymes can establish the complete structure of the prosthetic 

 group containing the growth factor. This type of investigation 

 is now being used to elucidate the structure of the active forms 

 of the growth factors pantothenic acid and biotin. Panto- 

 thenic acid-deficient cultures of organisms, which require this 

 substance as growth factor, are found to have impaired meta- 

 boHsm of acetic acid and impaired ability to carry out acetyla- 

 tion reactions. Thus deficient cultures of L. plantarum have 

 a greatly impaired ability to acetylate choline and this ability 

 can be restored by the addition of pantothenic acid to the 

 washed cells. Lipmann (see p. 34) has isolated a sulpha- 

 nilamide acetylase from animal tissues and shown that it 

 possesses a new prosthetic group, which he calls " coenzyme 

 A," which contains pantothenic acid. Concentrates of 

 coenzyme A have now been shown to act as the prosthetic 

 group of several enzyme preparations from bacteria and yeast 

 which catalyse reactions involving acetate or acetyl phosphate. 

 Thus a preparation which catalyses the condensation of 

 acetyl phosphate and oxalacetate to yield citrate is activated 

 by coenzyme A.' Coenzyme A has not yet been obtained in a 



