186 CARBON METABOLISM III 



P. chrysogenum is somewhat higher, particularly as to iron and 

 phosphorus, than that for growth (301). 



Corn steep liquor, a complex mixture of organic substances derived 

 by bacterial action on maize (339), probably has more than one role 

 in penicillin production; its principal function, however, appears to 

 be as a source of /?-phenylethylamine and related compounds (375), 

 which are specific precursors of benzylpenicillin. In the biosynthesis 

 of mixed penicillins, phenylacetic acid and its derivatives shift the 

 balance of natural penicillins toward benzylpenicillin (273). Similarly, 

 p-hydroxyphenylacetate increases the relative amount of p-hydroxy- 

 benzylpenicillin. Although only a small fraction of the phenylacetate 

 supplied is incorporated into the molecule (295), isotope studies have 

 shown that it is incorporated without dilution, i.e., that the intact 

 benzyl group enters as such into penicillin (38, 240, 467). 



Not only are close relatives of phenylacetic acid incorporated into 

 benzylpenicillin, but many other substituted acetic acids can serve as 

 precursors for new "biosynthetic" penicillins (37). Fatty acids, espe- 

 cially those which are not rapidly broken down by the fungus, are 

 incorporated into both natural and new penicillins (380, 516). 



The sulfur of penicillin can be supplied by inorganic sulfate; sulfur 

 of methionine, cysteine, cystine, or glutathione is, however, utilized in 

 preference to that of inorganic sulfate (490). 



The origin of the main part of the penicillin molecule has been 

 clarified by Arnstein and Grant (22). Using C 14 -labeled valine and 

 triply (N 15 , C 14 , S 35 ) labeled cystine, they have shown that: (1) cystine 

 — probably after conversion to cysteine — is incorporated directly into 

 the penicillin molecule, and (2) the remaining five carbons are sup- 

 plied by valine. This is consistent with earlier work on the incorpora- 

 tion of small molecules into penicillin (368, 489, 517). The molecule 

 of benzylpenicillin may be viewed, therefore, as made of three meta- 

 bolic units, one formed directly from phenylacetic acid, one directly 

 from cystine (probably via cysteine), and one supplied, directly or in- 

 directly, by valine. The steps by which these structures are combined 

 remain for future investigation. It is to be expected that energy is 

 necessary for penicillin formation from these units, and the inhibitory 

 effect of 2,4-dinitrophenol suggests that the energy is derived from 

 oxidative phosphorylations (449). 



The extensive studies on penicillin precursors have made it clear 

 that although a precursor may be specifically and directly incorporated 

 into a molecule, provision of the precursor does not necessarily in- 

 crease the final yield of product. It is only when the formation of the 

 precursor is so slow as to be rate-limiting to the total process that 



