Tracing heme in a living cell: hemoglobin degradation and
heme
            traffic in digest cells of the cattle tick Boophilus
microplus.
            Lara FA, Lins U, Bechara GH, Oliveira PL.
            Departamento de Bioquimica Medica, ICB, Universidade Federal
do
            Rio de Janeiro, Brazil.
            Heme is present in all cells, acting as a cofactor in
essential
            metabolic pathways such as respiration and photosynthesis.
            Moreover, both heme and its degradation products, CO, iron
and
            biliverdin, have been ascribed important signaling roles.
            However, limited knowledge is available on the intracellular
            pathways involved in the flux of heme between different cell
            compartments. The cattle tick Boophilus microplus ingests 100
            times its own mass in blood. The digest cells of the midgut
            endocytose blood components and huge amounts of heme are
            released during hemoglobin digestion. Most of this heme is
            detoxified by accumulation into a specialized organelle, the
            hemosome. We followed the fate of hemoglobin and albumin in
            primary cultures of digest cells by incubation with
hemoglobin
            and albumin labeled with rhodamine. Uptake of hemoglobin by
            digest cells was inhibited by unlabeled globin, suggesting
the
            presence of receptor-mediated endocytosis. After endocytosis,
            hemoglobin was observed inside large digestive vesicles.
            Albumin was exclusively associated with a population of small
            acidic vesicles, and an excess of unlabeled albumin did not
            inhibit its uptake. The intracellular pathway of the heme
            moiety of hemoglobin was specifically monitored using
            Palladium-mesoporphyrin IX (Pd-mP) as a fluorescent heme
            analog. When pulse and chase experiments were performed using
            digest cells incubated with Pd-mP bound to globin
            (Pd-mP-globin), strong yellow fluorescence was found in large
            digestive vesicles 4 h after the pulse. By 8 h, the emission
of
            Pd-mP was red-shifted and more evident in the cytoplasm, and
at
            12 h most of the fluorescence was concentrated inside the
            hemosomes and had turned green. After 48 h, the Pd-mP signal
            was exclusively found in hemosomes. In methanol, Pd-mP showed
            maximal emission at 550 nm, exhibiting a red-shift to 665 nm
            when bound to proteins in vitro. The red emission in the
            cytosol and at the boundary of hemosomes suggests the
presence
            of heme-binding proteins, probably involved in transport of
            heme to the hemosome. The existence of an intracellular heme
            shuttle from the digestive vesicle to the hemosome acting as
a
            detoxification mechanism should be regarded as a major
            adaptation of ticks to a blood-feeding way of life. To our
            knowledge, this is the first direct observation of
            intracellular transport of heme in a living eukaryotic cell.  
A
            similar approach, using Pd-mP fluorescence, could be applied
to
            study heme intracellular metabolism in other cell types.
            PMID: 16081607 [PubMed - in process]