EFFECTS ON PORPHYRIN SYNTHESIS 155 



acetate on the patterns of these metabolic systems have been reported. It 

 is clear that nitrate reduction must be inhibited fairly potently by iodoace- 

 tate. Sulfur oxidation by Thiohacillus thiooxidans is also markedly de- 

 pressed by iodoacetate, 60% at 0.01 mM and almost completely at 0.1 mM 

 (Vogler et al., 1942), growth being simultaneously abolished. Very similar 

 results were obtained by Iwatsuka et al. (1962), an iodoacetate concentra- 

 tion as low as 0.001 mM inhibiting 15%, but the COg fixation in the pre- 

 sence of sulfide is even more sensitive, being reduced 82% at 0.001 mM. 

 The incorporation of sulfate-S^^ into ChloreUa pyrenoidosa is inhibited when 

 iodoacetamide is added before the sulfate; when the iodoacetamide is added 

 5 min after the sulfate (which is a rather strange way to examine the effect 

 of an inhibitor) the total uptake is unchanged but the pattern of incorpo- 

 ration is altered (Schiff, 1959). Sidfated polysaccharide labeling is reduced 

 and more labeling occurs in lower molecular weight compounds. 



EFFECTS ON PORPHYRIN SYNTHESIS 



The synthesis of protoporphyrin from glycine and a-ketoglutarate in 

 chicken erythrocytes is inhibited 15% by 1 mM iodoacetamide and 63% 

 by 10 mM iodoacetamide, which indicates this part of the pathway to be 

 fairly resistant (Granick, 1958). The initial reactions of the synthesis, the 

 formation of r3-aminolevulinate from glycine, succinyl-CoA, and pyridoxal- 

 P, are not affected by 3.5 mM iodoacetamide (Gibson et al., 1958), so that 

 a more distal site must be visualized. This may well be the condensation of 

 r5-aminolevulinate to form porphobilinogen, since this is inhibited strongly 

 by 1 mM iodoacetamide (Gibson et al, 1955). The further condensation of 

 ]iorphobilinogen to coproporphyrin or protoporphyrin is not inhibited very 

 markedly by up to 10 mM iodoacetate, although there is some diversion 

 of the path from coproporphyrin to protoporphyrin (Rimington and Tooth, 

 1961), and the formation of uroporphyrinogen III from porphobilinogen is 

 only slightly depressed (Lockwood and Benson, 1960). The decarboxylation 

 of uroporphyrinogen to coproporphyrinogen is, however, well inhibited by 

 1.2 mM iodoacetamide (Mauzerall and Granick, 1958), so that where this 

 reaction is significant it may be a susceptible site for inhibition. The trans- 

 methylation from *S-adenosylmethionine to Mg-protoporphyrin to form a 

 methyl ester is actually stimulated slightly by 1-2 mM iodoacetate and 

 iodoacetamide (Gibson et al, 1963). The final incorporation of iron into 

 heme is moderately sensitive to iodoacetate in intact reticulocytes (59% in- 

 hibition by 1 mM) (Yoshiba et al, 1958), but in hemolysates from chicken 

 erythrocytes the inhibition is much less (9% by 1 mM) (Kagawa et al, 

 1959), indicating that in the former case the inhibition may be through 

 ATP depletion (2,4-dinitrophenol inhibits potently). From the limited evi- 

 dence one would conclude that in the synthetic pathway the most sensitive 



