ABSTRACT
Heme ferro protoporphyrin, quickly autoxidizes in aqueous solution. The binding of ferro protoporphyrin to the protein globin in its hydrophobic pockets enables the ferrous iron to form a reversible compound with molecular oxygen, oxyhemoglobin, and inhibits the autoxidation of ferrous heme to ferric heme, i.e., the formation of methemoglobin or ferrihemoglobin, but it does not fully prevent the latter reaction. The term “methemoglobinemia” was used, long before the existence of “normal” ferrihemoglobin was known, to describe abnormal states with easily detectable ferrihemoglobin contents in the blood. The early spectroscopic tests for ferrihemoglobin allowed the detection of such ferrihemoglobin contents as cause cyanosis. Following the early use of the term “ferrihemoglobinemia” to designate a pathological state, ferrihemoglobinemia should be defined as elevation of the ferrihemoglobin content of blood as causes pathological symptoms such as cyanosis or tissue hypoxia, since ferrihemoglobin does not bind oxygen reversibly as ferrohemoglobin does.
