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Molecular cloning and characterization of human age-related NADH oxidase (arNOX) proteins as members of the TM9 superfamily of transmembrane proteins

Author(s): Xiaoyu Tang | Debby Parisi | Bradley Spicer | Dorothy M. Morré | D. James Morré

Journal: Advances in Biological Chemistry
ISSN 2162-2183

Volume: 03;
Issue: 02;
Start page: 187;
Date: 2013;
Original page

Keywords: Age-Related NADH Oxidase (arNOX) | TM-9 Superfamily of Transmembrane Proteins; Molecular Cloning; Human Serum; Plasma and Body Fluids; Saliva

Age-related NADH oxidase (arNOX) proteins are superoxide-generating cell surface oxidases that increase in activity with age beginning at about 30 y. A soluble and truncated exfoliated form of the activity is present in blood and other body fluids. The activity was purified to apparent homogeneity from human urine and resolved by 2-D gel electrophoresis into a series of 24 to 32 kDa components of low isoelectric point. The purified proteins were resistant both to N-terminal sequencing and trypsin cleavage. Cleavage with pepsin revealed peptides corresponding to the TM9 family of transmembrane proteins. Peptide antisera raised to all five members of the human TM9 family sequentially blocked the arNOX activity of human saliva and sera. The soluble truncated N-terminus of the human homolog TM9SF4 was expressed in bacteria. The recombinant protein was characterized biochemically and exhibited arNOX activity. The findings identify five arNOX isoforms each of which correspond to one of the five known TM9 family members. The exfoliated soluble arNOX forms are derived from the 24 to 32 kDa N-termini exposed to the cell’s exterior at the cell surface. Each of the shed forms contain putative functional motifs characteristic of ECTO-NOX (ENOX) proteins despite only minimal sequence identity. Our findings identify arNOX as having functional characteristics of ENOX proteins and the TM9 superfamily of proteins as the genetic origins of the five known arNOX isoforms present in human sera, plasma and other body fluids.
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