The Aldehyde Oxidase Gene Cluster in Mice and Rats

Abstract

Mammalian molybdo-flavoenzymes are oxidases requiring FAD and molybdopterin (molybdenum cofactor) for their catalytic activity. This family of proteins was thought to consist of four members, xanthine oxidoreductase, aldehyde oxidase 1 (AOX1), and the aldehyde oxidase homologues 1 and 2 (AOH1 and AOH2, respectively). Whereas the first two enzymes are present in humans and various other mammalian species, the last two proteins have been described only in mice. Here, we report on the identification, in both mice and rats, of a novel molybdo-flavoenzyme, AOH3. In addition, we have cloned the cDNAs coding for rat AOH1 and AOH2, demonstrating that this animal species has the same complement of molybdo-flavoproteins as the mouse. The AOH3 cDNA is characterized by remarkable similarity to AOX1, AOH1, AOH2, and xanthine oxidoreductase cDNAs. Mouse AOH3 is selectively expressed in Bowman's glands of the olfactory mucosa, although small amounts of the corresponding mRNA are present also in the skin. In the former location, two alternatively spliced forms of the AOH3 transcript with different 3-untranslated regions were identified. The general properties of AOH3 were determined by purification of mouse AOH3 from the olfactory mucosa. The enzyme possesses aldehyde oxidase activity and oxidizes, albeit with low efficiency, exogenous substrates that are recognized by AOH1 and AOX1. The Aoh3 gene maps to mouse chromosome 1 band c1 and rat chromosome 7 in close proximity to the Aox1, Aoh1, and Aoh2 loci and has an exon/intron structure almost identical to that of the other molybdo-flavoenzyme genes in the two species.

Avian and Canine Aldehyde Oxidases

“…A similar arrangement of the four orthologous genes is present in rat (25). A striking conservation of exon structure is also evident when the AOX1, AOH1, AOH2, and AOH3 genes are compared with the mouse and rat XOR orthologs (1,25,27).…”

“…A similar arrangement of the four orthologous genes is present in rat (25). A striking conservation of exon structure is also evident when the AOX1, AOH1, AOH2, and AOH3 genes are compared with the mouse and rat XOR orthologs (1,25,27). Based on these as well as other observations, we proposed that all MOFE genes arose through one or more duplication events from a single ancestor with structural similarity to XOR (1).…”

“…The mouse AOX1, AOH1, AOH2, and AOH3 genes have strictly conserved exon structures and cluster in a small chromosomal region (aldehyde oxidase gene cluster) (25,27). A similar arrangement of the four orthologous genes is present in rat (25).…”

“…The AOX1 protein was eluted at 0.5 ml/min with a linear gradient from 0 to 1 M NaCl in 100 mM Tris-HCl, pH 7.5. The purification of AOX1 was monitored by RAL oxidizing activity and quantitative Western blot analysis (25) using an anti-bovine AOX1 antibody described previously (5), according to a chemiluminescence-based protocol (ECL, Amersham Biosciences). The antibodies do not cross-react with bovine XOR.…”

“…AOH3 is also peculiar, as the enzyme is selectively expressed in nasal mucosa (25). Given the recent identification of AOH1, AOH2, and AOH3, the corresponding physiological substrates and homeostatic roles are unknown.…”