One of two iron-storage substances present in animals, hemosiderin is a brownish-blue pigmented complex that stores unbound iron molecules that are not metabolically active. A type of macrophage or white blood cell, hemosiderin corrals inactive heme iron inside certain cells, such as bone marrow cells, spleen cells and liver cells, preventing the body from amassing high levels of unbound iron. Unbound iron, also known as free iron, can be toxic if left to build up in the body.
Hemoglobin, a molecule that is responsible for carrying iron safely in red blood cells, is a precursor to hemosiderin. After hemoglobin has released iron for various metabolic processes, it stores the remaining iron in two macrophages: ferritin and hemosiderin. Hemosiderin and ferritin are different in that ferritin stores iron in both plasma and cells; also, hemosiderin only stores inactive iron in cells. Ferritin is soluble while hemosiderin is insoluble.
Hemosiderin is considered a degraded and oxidized form of ferritin. That means it is a form of ferritin that has bound to oxygen molecules and has been broken down by the lysosomes, organelles that surround ferritin and use digestive acids to deteriorate it. The makeup of hemosiderin is believed to be iron oxide, various denatured proteins, and ferritin.
While free iron is the most dangerous form of iron, inactive iron stores are also considered toxic at abnormally high levels. Studies of patients with iron overload have found excess storage of inactive iron way beyond the typical 0.07 oz. (2 g) to 0.21 oz. (6 g) of stored iron found in most people. For example, patients diagnosed with hemochromatosis, a genetic disorder that prevents iron from metabolizing properly, may have 0.7 oz. (20 g) to 1.77 oz. (50 g) of stored iron, putting their organs at risk of malfunctioning due to iron poisoning.
Excess amounts of the bluish inactive iron in the liver can lead to cirrhosis, whereas too much iron storage in heart tissue can lead to cardiomyopathy. Too much iron in the pancreas contributes to diabetes mellitus. In joints, an overabundance of stored iron can cause polyarthropathy, which is a condition where joints are inflamed. High iron stores can also cause hyperpigmentation in the skin. A dearth of stored iron, however, can also cause ailments; patients with too little inactive iron storage may have iron deficiency anemia.
Detection of the brown or blue tint of hemosiderin in medical tests can reveal conditions involving injury to doctors. For example, the presence of the stored iron in urine can indicate intravascular hemolysis, which is a destruction of red blood cells. Hemorrhages in the brain or elsewhere in the body can also be detected by the presence of the bluish-brown iron in surrounding tissues.