DAVID SHEMIN 



glycine, no longer attached to the carboxyl group, is utilized for 

 the synthesis of porphyrins, the ureido groups of purines, the 

 j8-carbon atom of serine, and for methyl groups. This metabolic 

 pattern is similar to that of the so-called "Ci" compounds, with 

 the exception that the latter cannot substitute for glycine in 

 porphyrin synthesis. It would seem therefore that these ap- 

 parently unrelated compounds (porphyrins, purines, serine, and 

 methyl groups) have one common feature, namely, the partici- 

 pation of the a-carbon atom of glycine for their synthesis. 



CH- 



CHp 

 CH 



CH- 



An' 



HC6 



v^ 



= C- 

 ct 



H 



CHo 

 II "^ 



CH 



H • 



CH>9 



CH, CH; 

 I 



CHp 



I 



COOH 



CHp CH - 



CHo 



I 



COOH 



NH2-CH2-COOH 

 Glycine-2-c''* 



NHg-CHg-CO-CHg-CHg-COOH 

 6- Aminolevulinic Acid-5-c'^ 



PROTOPORPHYRIN IX 



Fig. 2. The carbon atoms of protoporphyrin which arise from the a-carbon 

 atom of glycine and from the 5-carbon atom of 5-aminoleviiUnic acid. 



It would appear reasonable from a unitarian approach, 

 therefore, to consider the possibility that glycine is metabo- 

 lized via a pathway in which intermediates are produced which 

 then can be utilized for the synthesis of these different com- 

 pounds. In an attempt to unify the reactions of glycine we have 

 postulated a series of reactions, called the succinate-glycine 

 cycle (Figure 1). This pathway for glycine metabolism sug- 

 gested itself from a study of the mechanism of porphyrin syn- 

 thesis. In this pathway it is postulated that "active" succinate 

 condenses on the a-carbon atom of glycine to give rise to a- 

 amino-j8-ketoadipic acid. This j3-keto acid decarboxylates to 

 give rise to 5-aminolevulinic acid, which can be utilized for 

 porphyrin synthesis or further metabolized in such a manner 



520 



