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Porphyrin fluorescence similar to that seen in iron-deficient erythrocytes
is also observable in erythropoietic porphyria, in which porphyrins are
overproduced, and in poisoning with lead and other heavy metals, which block
ferrochelatase-catalyzed incorporation of iron into heme.
In iron deficiency and lead poisoning, the predominant fluorescent porphyrin
is zinc protoporphyrin, with an emission maximum around 594 nm, and, while
this has been measured by flow, (see the 3rd Edition of Practical Flow
Cytometry for details), it's pretty weak fluorescence, even when measured
near the emission maximum and excited at 441 nm, where the absorption is
much higher than at 488 nm. I wouldn't expect to see much in the green
region in which thiazole orange is typically measured with 488 nm
excitation; autofluorescence here has generally been ascribed predominantly
to flavins.
In any case, if what is being observed in the hemoglobin E patient is
porphyrin fluorescence, it should be at least as strong if not stronger in
the orange (PE) channel than in the green.
Don't you just love the really good mysteries that come up on the mailing list?
-Howard
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