344 



CELLS, TISSUES, AND ORGANISMS 



HOOC 



CH2 COOH 



CH2 CH2 



i— i 



HC C-CHzNhz 



Porphobilinogen 



I 



(CH2)2COOH CH2COOH 



-(CH2)2C00H 



HOOCCH2 



H00CCH2^ A X VCHzCOOH 

 (CH2)2C0OHfCH2)2C00H 



H00C(CH2)2 CH3 CHi 

 HOOCCH2 A A ACH2COOH 



Mc+hylation 



HOOCCH2 



(CH2)2C00H 

 Co 



CH2COOH 

 Chi 



CH3(CH2)2 COOH (CH2)2COOH 



dehydroqenation 

 Formo+ion of link 

 between A+D nnc^s 

 amida+ion •, 

 couplina with 

 nucleotide 



Vi-^-amin 612 



Figure 8. Suggested steps in biosynthesis of Vitamin B^g- 



resembles the A ring of riboflavin attached to a portion of the B ring. 

 This nucleotide is linked to the ring through a propionamide side chain 

 esterified to phosphoric acid. Animals are unable to link together the 

 component parts of vitamin B12, but certain bacteria can readily add 

 various nucleotides to the porphyrin-like portion of the vitamin B12 

 molecule. In this way a number of analogs of vitamin B12 have been 

 produced, and a few of these occur naturally in bacterial systems. 



Animals apparently can convert vitamin B12 to the coenzymatic 

 form of the vitamin that has been described by Barker and co-workers 

 (1960). 



Ascorbic Acid. Ascorbic acid can be formed in plants and in all 

 animals except guinea pigs and primates. It has been shown by labeling 

 experiments with rats that either D-glucose or D-galactose give rise to 

 L-ascorbic acid, and that the carbon skeleton remains intact. 



