The Biosynthesis of Porphyrins 257 



is also converted to formic acid. Thus, it has been demonstrated that 

 glycine is metabolized via this pathway. 33 



The condensation of glycine with "active" succinate provides a path- 

 way whereby glycine can be oxidized to carbon dioxide and the inter- 

 mediates produced in the cycle drawn off for the synthesis of other 

 compounds. This is similar to the citric acid cycle, in which another 

 2-carbon compound is oxidized to carbon dioxide and intermediates 

 are produced which can be drawn off for synthesis. In the succinate- 

 glycine cycle, succinate is the catalyst instead of oxaloacetate. 



This work was supported by grants from the National Institutes of 

 Health, United States Public Health Service (RG-1128(C6) ), from 

 the American Cancer Society on the recommendation of the Committee 

 on Growth of the National Research Council, from the Rockefeller 

 Foundation, and from the Williams-Waterman Fund. 



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