160 1. MALONATE 



mination of the porphyrin labeling (Shemin and Kumin, 1952). Succinate- 

 C^^OO" when oxidized through the cycle gives rise to a-carboxyl-labeled 

 a-ketoglutarate and hence to unlabeled succinyl-CoA; therefore, no porphy- 

 rin labeling should result from this pathway. However, succinate-C"00~ 

 could also directly form succinyl-CoA, which in this case would be labeled 

 and C^* would be found in porphyrin. On the other hand, succinate-C^^Hg 

 would form labeled succinyl-CoA by both pathways. If it is assumed that 

 malonate inhibits the oxidation of succinate and the cycle pathway only, 

 malonate should not inhibit any porphyrin labeling after incubation with 

 succinate-C^*00~, but should inhibit appreciably the porphyrin labeling 

 from succinate-C^^Hg. This was found by Shemin and Kumin, as the aver- 

 aged results in the accompanying tabulation show (figures are counts/ 



Hemin Control Malonate 20 mif % Change 



From succinate-C'^OO- 215 210 - 2 



From succinate-Ci^Hj 1025 421 -59 



minute for intact erythrocytes). It would appear that both pathways are 

 operative in these cells. The failure of malonate to increase the porphyrin 

 labeling from succinate-C^*00~ is rather surprising because one might 

 expect malonate to divert some of the succinate from the cycle into the 

 formation of succinyl-CoA. It is possible that this effect is somewhat coun- 

 teracted by an inhibition on succinyl-CoA synthetase. 



Further information on porphyrin synthesis and the effects of malonate 

 were obtained by Wriston et al. (1955) by the use of labeled acetate. Different 

 malonate effects were obtained when methyl-labeled and carboxyl-labeled 

 acetate were incubated with glycine, the inhibition of porphyrin labeling 

 being much greater with the former (see tabulation). This is the expected 



Hemin Control Malonate 20 mM % Change 



From acetate-Ci^Ha 376 174 -54 



From acetate-C'^OO- 72.5 67 - 8 



result, because the formation of labeled succinyl-CoA from acetate-C^*00~ 

 does not involve the complete cycle and the C^* pathway does not go through 

 the succinate oxidation step, whereas porphyrin labeling from acetate- 

 C^^Hg depends on the operation of the entire cycle (except for the contri- 

 bution from the y-C atom of a-ketoglutarate). Furthermore, the labeling 



