Coated pit formation: a membrane function involved in the regulation of cellular iron uptake.

Diferric-transferrin induces a marked increase in the number of coated pits on the reticulocyte membrane. This increase is followed by a decline, often to below the initial number. Since a good correlation was found between the rate of iron uptake and the number of coated pits, but not between the rate of transferrin recycling and the coated pit count, it is likely that coated pit formation is necessary for the removal of iron from transferrin. The decline in the number of transferrin-induced coated pits was observed only when haem synthesis was undisturbed, indicating that the accumulation of intracellular haem inhibits coated pit formation. Based on these results we suggest that haem regulates the rate of iron uptake by inhibiting iron removal rather than receptor recycling.