THE LOCALIZATION OF FERRITIN IN INSECTS

The location of holoferritin, that is ferritin with an iron filled core visible by transmission electron microscopy, has been studied in iron rich insect tissues. We have found that holoferritin is present in the ER and secretory pathway of all species studied, representing nine families from six orders. Cisternal ferritin has an advantage over cytosolic ferritin for coping with an iron overload in that it can be secreted to the outside. Although in theory the malpighian tubules could secrete holoferritin apically and so extrude unwanted iron from the body, this was only observed in the gut. Holoferritin was found in the midgut apical Golgi complexes of all species surveyed. The exocytosis of iron loaded ferritin by the midgut is probably the main route for iron excretion by insects. In tissues without an external face for secretion, the abundance of ferritin in the ER without concentration in secretory vesicles suggests that it is retained or only moved slowly from the ER. Most insects therefore differ from vertebrates in their iron metabolism in that they rely on vacuolar rather than cytosolic ferritin for intracellular iron buffering. Cytosolic holoferritin occurred in some species but was only found abundantly in xylem feeding Homoptera.