252 Essays in Biochemistry 



contained less C 14 than those of the controls made either from C 14 - 

 labeled glycine or succinate. 17 These results, which are in full agree- 

 ment with the hypothesis, can also be explained, however, by the possi- 

 bility that S-aminolevulinic acid is acting, not as a diluent, but as an 

 inhibitor of heme synthesis. To rule out the latter possibility, the 

 S-aminolevulinic acid added in experiment 2 (Table 2) was labeled 

 with N 15 . It can be seen that, whereas the incorporation of C 14 from 

 the glycine was lowered, there was a comparatively large incorporation 

 of N 15 into the porphyrin, thus demonstrating that the lowered C 14 

 activity of the hemin sample was due to dilution rather than inhibition. 

 Further proof that ^-aminolevulinic acid is a result of the condensation 

 of glycine and succinate was obtained by incubating red blood cell 

 hemolyzates with glycine-2-C 14 and unlabeled 8-aminolevulinic acid, 

 and subsequently isolating the S-carbon atom. In such an experiment 

 it was found that the formaldehyde liberated upon periodate oxidation 

 of a crude fraction containing S-aminolevulinic acid was highly radio- 

 active. 



More rigorous proof that S-aminolevulinic acid is indeed the pre- 

 cursor for porphyrin synthesis was obtained by degrading a hemin 

 sample synthesized from S-aminolevulinic acid-5-C 14 and from S- 

 aminolevulinic acid-l,4-C 14 . The S-carbon atom of the former com- 

 pound should label the same carbon atoms of protoporphyrin as those 

 which we have previously found to arise from the a-carbon atom of 

 glycine, since according to the hypothesis the latter carbon atom is 

 the biological source of the S-carbon atom of S-aminolevulinic acid. 

 Furthermore, the S-aminolevulinic acid-l,4-C 14 should label the same 

 carbon atoms of protoporphyrin found to arise from the carboxyl 



Table 3. Distribution of C' 4 Activity in Protoporphyrin Synthesized 

 from 8-AminoIevuIinic Acid-5-C 14 and from Glycine-2-C 14 



Molar Activity (%) in Fragments 

 of Porphyrin Synthesized from 



