Haem controls the rate of haem synthesis in erythroid cells by inhibiting iron incorporation from transferrin. The present results indicate that haem primarily inhibits the release of iron from transferrin subsequent to transferrin endocytosis and that the inhibition of transferrin endocytosis caused by relatively high concentrations of haem is a secondary effect. Low concentrations of haem (10-25 microM) significantly inhibit reticulocyte iron uptake and to a greater extent its incorporation into haem, but do not inhibit either the initial rate of transferrin uptake or its internalization by the cells.

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