title: An ascorbate-reducible cytochrome b561 is localized in macrophage lysosomes.
abstract: Cytochromes b561 (Cyts b561) are a family of intrinsic membrane proteins involved in ascorbate-mediated transmembrane electron transport. The chromaffin granule Cyt b561 (CGCytb) is believed to transport electrons donated by extravesicular ascorbate (ASC) across the membrane to intravesicular monodehydroascorbate (MDA) supporting catecholamine synthesis in neuroendocrine tissues. Another isoform, the duodenal Cyt b561 (Dcytb), was reported to have ferric reductase activity, possibly facilitating intestinal iron uptake. Herein, a new Cyt b561 homologue, LCytb (for lysosomal Cytb561) was found expressed in the late endosomal-lysosomal membrane. LCytb shared high sequence similarity with CGCytb (45% identity) and Dcytb (42% identity). Moreover, four heme-coordinating His residues, and putative ASC and MDA binding sites were highly conserved. Recombinant LCytb exhibited an ASC-reducible b-type Cyt absorbance spectrum with alpha-band maximum at 561 nm in the spectrum of the reduced protein. Northern blots and Western blots revealed that LCytb was predominantly expressed in lung, spleen, thymus, testis and placenta. In situ hybridization and immunofluorescence studies further demonstrated that the protein was expressed in the alveolar macrophages of the lung, in the white pulp of the spleen, widespread in the thymus, and in the Sertoli cells of the testis. Sequence analysis indicated the presence of a (DE)XXXL(LI)-type signal in the C-terminal of the protein, predicting a late endosomal-lysosomal subcellular localization. This localization was confirmed by double labeling experiments in RAW264.7 and 293 cells, stably transfected with LCytb.
authors.affiliation: Department of Biochemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA.
authors.forename: De-liang
authors.initials: DL
authors.lastname: Zhang
authors.forename: Dan
authors.initials: D
authors.lastname: Su
authors.forename: Alajos
authors.initials: A
authors.lastname: Bérczi
authors.forename: Amy
authors.initials: A
authors.lastname: Vargas
authors.forename: Han
authors.initials: H
authors.lastname: Asard
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001205:Ascorbic Acid / Q000378:metabolism*; D015153:Blotting, Western; D002460:Cell Line; D003573:Cytochrome b Group / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D017403:In Situ Hybridization; D008247:Lysosomes / Q000201:enzymology*; D008264:Macrophages / Q000201:enzymology*; D051379:Mice; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D017386:Sequence Homology, Amino Acid
chemical_list: D003573:Cytochrome b Group; C023646:cytochrome b561; D001205:Ascorbic Acid