To the Editor.—
It is well known that hemoglobin-derived, native formalin pigment (acid formaldehyde hematin) does not give a positive histochemical reaction for iron. However, this does not indicate that formalin pigment is an iron-free derivative of hemoglobin as claimed by Drs. Ackerman and Pennys in the Archives (102:670, 1970). Gomori1 stated that a number of well-defined iron-containing compounds, such as hemoglobin, malaria pigment, and formalin pigment, which do not show any reaction with the Prussian blue method for iron can be made positive by destroying the organic part of the molecule ("unmasking" the iron). For this "unmasking," Gomori1 recommended the use of hydrogen peroxide since it produces an insoluble ferric oxide, the nondiffusibility of which being important in its histochemical localization. For the purpose of bleaching formalin pigment, Lillie2 has recommended 30 minutes' treatment in 3% hydrogen peroxide. I have found that the pigment is usually
Holmes EJ. REMARKS ON FURTHER PROPERTIES OF FORMALIN PIGMENT. Arch Dermatol. 1971;103(5):565–566. doi:10.1001/archderm.1971.04000170099028
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