PORPHYRINS 



methylene bridge carbon atoms rather than methene bridge 

 carbon atoms, and consequently uroporphyrin and copropor- 

 phyrin are oxidized products of the intermediates. 



If the proposed succinate-glycine cycle is a pathway of 

 glycine metabolism embracing the reactions known for the 

 a-carbon atom of glycine, then the 5-carbon atom of 5-aminolev- 

 ulinic acid should have the same metabolic spectrum. To 

 test the hypothesis, 5-aminolevulinicacid-5-C^^ was injected into 

 a duck, and hemin, guanine, and adenine were isolated from the 

 red blood cells ; serine and the methyl group of methionine were 

 obtained from the plasma proteins, and uric acid was isolated 

 from the excreta. 



It was found that the ureido groups of the purines (19), the 

 jS-carbon atom of serine (9), and the methyl group of methionine 

 (8) contained C^^. These experimental results, which support 

 the existence of the succinate-glycine cycle, are to be considered 

 as only preliminary in nature, since quantitative aspects have 

 not yet been completely explored. Quantitative evaluation is 

 difficult in whole animals, since cells are not very permeable 

 to 5-aminolevulinic acid and the compound is very unstable as 

 the free amino acid. In an experiment in the bird the C^'* 

 activity of both the hemin and the ureido groups of the purines 

 synthesized from 5-aminolevulinic acid-S-C^^ was 2.5 times 

 greater than in those samples synthesized in a comparable ex- 

 periment in which glycine-2-C^'* was the substrate. This may 

 be a minimal difference if one considers the experiments in 

 vitro (see Table II) in which ihe C^^ activity of the hemin syn- 

 thesized from 5-aminolevulinic acid was about 65 times greater 

 than that synthesized from glycine. However, it would appear 

 from the above results that the 5-carbon atom of 5-aminolevu- 

 Imic acid has the same metabolic pattern as that of the a-carbon 

 atom of glycine and likewise as that of the "Ci" compounds be- 

 yond the porphyrin synthesis. Further support for this con- 

 clusion is our finding that while bothglycine-2-C^'* and 5-amino- 

 levulinic acid-5-C'^ gave rise to radioactive formate in the urine 

 of a rat, the formate arising from 5-aminolevulinic acid was about 



533 



