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Murine erythroleukaemia (MEL) cells are virus-transformed erythroid precursor cells that, when induced to differentiate by dimethyl sulphoxide (DMSO), will initiate haem biosynthesis by the induction and synthesis de novo of all of the enzymes of the haem-biosynthetic pathway. The activities of porphobilinogen (PBG) deaminase (EC 4.3.1.8), coproporphyrinogen oxidase (EC 1.3.3.3), protoporphyrinogen oxidase (EC 1.3.3.4), ferrochelatase (EC 4.99.1.1) and NADH:ferric iron reductase, as well as the synthesis of the enzyme ferrochelatase and the levels of excreted porphyrins, were monitored during DMSO-induced differentiation of MEL cells in culture. The data demonstrate that PBG deaminase and protoporphyrinogen oxidase activities rise rapidly and early, in comparison with ferrochelatase activity, which rises more slowly, and coproporphyrinogen oxidase activity, which decreases by 60% within 24 h of induction before returning to initial levels by 72 h. NADH:ferric iron reductase activity increases slightly, but is always present at levels higher than needed for haem synthesis. Total immunoprecipitable ferrochelatase also rises slowly and parallels the increase in its activity, suggesting that it is not synthesized early in a slowly processed precursor form. Examination of culture media demonstrated that, whereas excretion of protoporphyrin and coproporphyrin occurs within 24 h of induction, coproporphyrin is excreted in amounts 4-15 times greater than protoporphyrin.

Coproporphyrinogen oxidase (EC 1.3.3.3), protoporphyrinogen oxidase (EC 1.3.3.4), and ferrochelatase (EC 4.99

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