342 CELLS, TISSUES, AND ORGANISMS 



actions involving coenzyme A. The next step, the hydroxymethylation 

 of ketovahne, can be brought about by formaldehyde in the presence 

 of boiled extracts of E. coli. However, it seems more likely that a folic- 

 acid system involving serine may be responsible for the biosynthetic 

 mechanism in living bacterial cells. The steps are summarized in 

 Figure 6. 



The formation of beta-alanine can occur from decarboxylation of 

 aspartic acid, from transamination of formylacetic acid, or from pro- 

 pionyl CoA via acrylyl CoA and malonic acid semialdehyde (Stadt- 

 man, 1955 ) . 



According to present concepts of the biological synthesis of pan- 

 toic acid, it seems evident that pantothenic acid participates in its own 

 biosynthesis. 



Folic Acid. The distinguishing feature of the molecule of folic acid 

 is its pteridine ring. This has a biogenetic origin somewhat similar to 

 that of the purine ring. Very large quantities of pteridines are synthe- 

 sized by butterflies for use as wing pigments. The biosynthesis of leu- 

 copterin, the white pigment of the wings of the cabbage butterfly, was 

 studied by Weygand and Waldschmidt ( 1955 ) , who administered 

 various tagged compounds to the larvae of this insect and measured 

 the labeling of leucopterin in the adults. By this means it was found 



CHj CH3 



CH-CH-COOH )> CH-CO-COOH — — 



'1 • FA 



CH3NH2 CH3 



Vo I i n e Keto va I i ne 



CH3 CH3 



CH2OH-C-CO-COOH > CH2OH-G— CH-COOH 



CHj CHjOH 



NH2CH2CH2COOH 



CH5 

 CH2OH- C-CHOH-CONHCH2CH2COOH 

 CH3 



Pantothenic acid 



Figure 6. Biological synthesis of pantothenic acid in E. coli. 



