Ascorbic acid and iron metabolism: alterations in lysosomal function.

Iron is essential to cell metabolism but promotes free radical damage to membranes and lipids. Therefore, excess intracellular iron is stored within the shell of hollow ferritin molecules until needed for metabolic use. Ascorbate retards ferritin degradation and increases iron bioavailability. The vitamin stabilizes the iron cores of ferritin in cells prelabeled with 59Fe. [35S]Methionine labeling demonstrates that this enhanced stability of the iron cores results from delayed degradation of the ferritin shells. Subcellular fractionation of 59Fe-labeled cells by use of a Sepharose CL-6B column shows that ascorbate significantly delays the shift of ferritin label from the cytosolic to the lysosomal compartment. Monomeric ferritin shells in the cytoplasm gradually form clusters that bind to lysosomes. Single ferritin shells do not. Ascorbate does not affect the conversion of cytoplasmic ferritin monomers to clusters but greatly retards the autophagic uptake of ferritin clusters into lysosomes.