104 growth: synthesis of bacterial protoplasm 



pure state but there is little doubt tbat it is a compound con- 

 taining pantothenic acid as part of its structure. The position 

 of biotin is very similar : deficient culture studies have shown 

 that biotin-deficiency results in impairment of oxalacetate 

 formation from pyruvic acid and COg, and in decreased activity 

 of aspartic acid and serine deaminases (see p. 163). Cell-free 

 preparations of aspartic acid deaminase can be activated by 

 biotin and adenylic acid; both factors are necessary and it is 

 fairly certain that the preparations contain a second enzyme 

 which brings about the synthesis of a co-aspartase from them. 

 A combined form of biotin has been isolated from yeast 

 extracts and this material will act as co-aspartase. Its 

 structure has not yet been determined. 



In most of these cases we find that the biologically active 

 structure (= prosthetic group) is a more complex molecule 

 than the growth factor. This can be explained by saying that 

 the synthesis of the growth factor is more difi&cult for the 

 organism to accomplish than that of the rest of the prosthetic 

 group but, on the mutational explanation of variation, it is 

 probably more correct to say that mutations involving loss of 

 synthetic ability towards the growth factor give the organism 

 a greater energetic advantage (or " selection pressure ") in a 

 complex medium than those involving loss of ability to 

 synthesise the rest of the molecule. Whatever the explanation, 

 it does not follow that all strains of an organism which is 

 nutritionally exacting towards, say, pantothenic acid will 

 display a disability towards the whole molecule. Some 

 organisms can synthesise pantothenic acid if they are supplied 

 with jS-alanine. Others, which apparently require biotin, can 

 synthesise this factor if supplied with the pimelic acid part 

 thereof. In these cases, the lost enzymes are concerned with 

 the synthesis of the j8-alanine portion of pantothenic acid or 

 the pimelic acid portion of biotin respectively. 



Many growth factors have been discovered by analysis of 

 complex "satisfactory" media, but one, j9-amino-benzoic acid , 

 was first discovered as an antagonist to the drug sulphanila- 

 mide (seep. 113) and then shown to be a growth factor for, first, 



