1. A method, based on the uptake of radioiron into heme, is described for the measurement of heme synthesis in a hemolysate of chicken erythrocytes.

2. The addition of glycine, δ-aminolevulinic acid, porphobilinogen or protoporphyrin 9 to the system augmented heme synthesis.

3. Citrate potentiated heme synthesis in the presence of glycine, but had no effect when porphobilinogen was added. Succinate, in the presence of glycine did not enhance heme synthesis.

4. The addition of coproporphyrin I or III, or uroporphyrin I or III, did not augnment the uptake of radioiron into heme. The addition of mesoporphyrin 9 or hematoporphyrin 9 enhanced the uptake of radioiron. These studies are iterpreted as suggesting that protoporphyrin, but not uroporphyrin or coproporphyrin, is a precursor of heme. For reasons discussed, further work will be necessary to determine if mesoporphyrin and hematoporphyrin are precursors of protoheme.

5. The synthesis of heme was inhibited by the addition of malonate or lead. Evidence is presented that both of these substances affected heme synthesis at several different levels, particularly the formation of δ-aminolevulinic acid and the incorporation of iron into protoporphyrin.

6. Plasma extracts from chickens, made anemic by bleeding or by the administration of phenylhydrazine, did not potentiate heme synthesis.

7. Normal, nonreticulated, human erythrocytes failed to synthesize heme in the presence of glycine, δ-aminolevulinic acid, porphobilinogen on protoporphyrin 9.

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