go INTERMEDIARY METABOLISM AND GROWTH I 



incorporated into carbon atoms 3 and 5 of the pyrrole rings. Malonate inhibited 

 the labeUing of protoprophyrin when acetate-2-'"*C was the substrate but not 

 when acetate- 1 -''^C was the substrate (Wriston et al., 1955). 



The product of the condensation ofsuccinyl-CoA and glycine is a-amino-[3-keto- 

 adipic acid. The latter substance is converted to S- aminolevulinic acid by loss of 

 the a-carboxyl group, the carboxyl atom of the glycine substrate (Fig. 42). When 

 glycine-s-'^^C was incubated with the hemolyzed duck erythrocytes, 8-amino- 

 levulinic acid-5-''*C could be isolated from the medium. Labelled S-amino- 

 levulinic acid was also obtained when '"'C-succinyl-CoA was the substrate. Fur- 

 thermore, non-labelled S-aminolevulinic acid markedly inhibited the incorporation 

 of labelled glycine- or succinyl-CoA into protoporphyrin ( Shem in and Russell, 1953). 



Particle free extracts of the erythrocyte hemolysates contained the enzymes for 

 the synthesis of protoporphyrin from S-aminolevidinic acid labelled with either 

 ^"•C in the fifth carbon or with '^N (Schmid and Shemin, 1955). Furthermore, 

 extracts from chicken erythrocytes, spinach, Chlorella cells or a Tetrahymena 

 strain were also capable of effecting this conversion. The precursor pyrrole was 

 identified as porphobilinogen, a compound isolated from the urine of patients 

 with acute porphyria (Granick, 1954). S- aminolevulinic acid itself was also isolated 

 from the urine of men with acute porphyria and from normal men (Mauzerall and 

 Granick, 1956; Fig. 42). The synthesis of labelled porphobilinogen from labelled 

 S-aminolevulinic acid or the diethylester of a-amino-^-ketoadipic acid was also 

 demonstrated in experiments with ox liver or duck erythrocyte enzymes. More- 

 over, the porphobilinogen had twice the molar specific activity of the precursor 

 ^-aminolevulinic acid. Labelled porphobilinogen was in turn incorporated into 

 protoporphyrin by erythrocyte enzymes or by colorless extracts of Chlorella cells. 

 The enzymes for the synthesis of porphobilinogen from S-aminolevulinic acid were 

 also found in mature human erythrocytes, cells which are incapable of effecting 

 the complete synthesis of heme in vitro. 



The side chains of pyrrole rings A and B of protoporphyrin differ from those of 

 rings C and D (Fig. 41). The mechanism by which the side chains are decarbo- 

 xylated in the synthesis of protoporphyrin is not known as yet. There are indica- 

 tions that uroporphyrin, coproporphyrin, and protoporphyrin are formed inde- 

 pendently from porphobilinogen (Schwartz, 1955). Shemin et al. have suggested 

 that a tripyrrole methane compound is an intermediate in the synthesis of the 

 tetrapyrroles (Fig. 42) (Shemin et al., 1955). Cleavage of the tripyrrole methane 

 at A or B would produce a dipyrrole inethane compound of structure (A) or (B) 

 respectively and a monopyrrole. The condensation of two molecules of dipyrrole 

 methane (A) would produce a tetrapyrrole of the porphyrin I series whereas a 

 tetrapyrrole of the porphyrin III series wovdd arise by a condensation of one 

 molecule of (A) with one molecule of dipyrrole methane (B). It may also be seen 

 that the above mechanism necessitates the loss of one of the three aminomethyl 

 side chains in the formation of a protoporphyrin III compound from the conden- 

 sation of the A and B dipyrroles. Consistent with the suggested mechanism was 

 the finding that radioactive formaldehyde was formed as a result of conversion 

 of porphobilinogen labelled with ^'*C on the aminomethyl group to porphyrins 

 either by heating under acid conditions or enzymatically in cell free extracts. 



