Shemin and coworkers (14) showed that the a-carbon and nitrogen atoms of glycine were precursors of specific atoms of the heme molecule. This was substantiated by Muir and Neuberger (5, 6). Other carbon atoms were shown to come from acetate (5-7) or succinate (8). The nature of the intermediate precursors synthesized from these metabolites remained unknown until the structure for urinary porpho...