Three mammalian cytochromes b561 are ascorbate-dependent ferrireductases

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Cytochromes b(561) are transmembrane proteins present in eukaryotic cells.
• Mammalian cytochromes b(561) include chromaffin granule cytochrome b, duodenal cytochrome b, and lysosomal cytochrome b.
• Iron metabolism is critical for cellular function, involving iron uptake and reduction.

Purpose of the Study:

• To investigate the ferrireductase activity of mammalian cytochromes b(561) in iron metabolism.
• To identify key amino acid residues essential for the function of lysosomal cytochrome b.
• To demonstrate the role of intracellular ascorbate in cytochrome b(561) activity.

Main Methods:

• Expression of mammalian cytochromes b(561) in a Saccharomyces cerevisiae Deltafre1Deltafre2 mutant.
• Assay of plasma membrane ferrireductase activity using intact yeast cells with FeCN and Fe(3+)-EDTA.
• Site-directed mutagenesis of lysosomal cytochrome b to identify critical amino acids.

Main Results:

• Expression of all three mammalian cytochromes b(561) rescued the growth defect of the yeast mutant in iron-deficient conditions.
• All tested cytochromes b(561) exhibited significant FeCN and Fe(3+)-EDTA reductase activities dependent on intracellular ascorbate.
• Mutations in specific histidine, tyrosine, and arginine residues of lysosomal cytochrome b abrogated its FeCN reductase activity.

Conclusions:

• Mammalian cytochromes b(561) possess ascorbate-dependent transmembrane ferrireductase activity.
• Specific amino acid residues (His, Tyr, Arg) are critical for lysosomal cytochrome b function, potentially affecting heme coordination or substrate binding.
• These findings highlight the involvement of cytochrome b(561) family proteins in cellular iron metabolism.