The biochemistry of ferritin.

The researchers investigated the biochemical properties of ferritin, a protein responsible for iron storage in the body. They identified its distribution and structure, noting variations in different species and tissues.

Distribution and Isolation of Ferritin

• The researchers found ferritin to be primarily located in the spleen and liver of mammals, with lesser amounts in the bone marrow. Ferritin was also found in other tissues like intestinal mucosal cells, kidney, heart, testes, skeletal muscle and placenta.
• They confirmed the presence of ferritin in human serum, and noted that serum ferritin levels seem to be an indicator of the body’s iron stores.
• Ferritin was found not only in invertebrates and fish, but also in plants and a specific fungus, Phycomyces. The amenability of horse spleen ferritin to isolation made it the most studied.
• Technical challenges in isolating ferritin from other species and tissues were also discussed.

Structure of Ferritin

• Upon isolation, ferritin consists of a population of molecules that contain from zero to 4500 atoms of iron. The average iron content for horse spleen ferritin is about 3700 atoms.
• The researchers described the structure of the iron, present as ferric hydroxyphosphate, as occupying a central area and enclosed within a shell of protein.
• The iron-free protein version, apoferritin, has a stable molecular weight of 440,000 and consists of 24 identical subunits each with a molecular weight of 18,500. Apoferritin is notably stable under certain pH conditions, but can be dissociated into subunits under pH extremities or particular treatments.
• Reassociation of apoferritin from subunits occurred in the absence of iron. The researchers suggested that hydrophobic interactions, hydrogen bonds and possibly salt bridges are key forces in the structure of apoferritin.

Existence of Different Ferritins

• The researchers noted the occurrence of different, tissue-specific ferritins within the same species.
• In human bone marrow, two types of ferritin were found, one corresponding to spleen ferritin and the other to reticulocyte ferritin.
• Furthermore, they found variations in the amino acid composition and tryptic peptide patterns in the apoferritins isolated from human and horse liver and spleen
• However, the researchers acknowledged that these structural differences do not necessarily indicate functional differences in different tissues.