The Biological Function of Cauxin, a Major Urinary Protein of the Domestic Cat (Felis catus)
A major protein component of domestic cat urine is the carboxylesterase family member termed cauxin. Cauxin is secreted into the urine from the proximal straight tubular cells of the kidney, and the level of cauxin excretion is species-, sex-, and age-dependent. Cauxin is excreted in large amounts in the closely related members of the Felidae lineage, the cat (Felis catus), bobcat (Lynx rufus), and lynx (Lynx lynx). Male and female immature cats begin excreting cauxin about 2.5 months after birth, and excretion levels increase with age. In mature cats, cauxin excretion is significantly higher in intact males than in castrated males or female cats. The physiological function of cauxin is to provide species-, sex-, and age-dependent regulation of 2-amino-7-hydroxy-5,5-dimethyl-4-thiaheptanoic acid (felinine) production. Cauxin hydrolyzes the peptide bond of the felinine precursor, 3-methylbutanol-cysteinylglycine, to produce felinine and glycine. The sulfur-containing volatile compounds, 3-mercapto-3-methyl-1-butanol, 3-mercapto-3-methylbutyl formate, 3-methyl-3-methylthio-1-butanol, and 3-methyl-3-(2-methyl-disulfanyl)-1-butanol, are identified as species-specific odorants and candidates of felinine derivatives from the headspace gas of cat urine. These cat-specific volatile compounds may represent pheromones used as territorial markers for conspecific recognition or reproductive purposes by mature cats. The elucidation of cauxin-dependent felinine production provides new evidence for the existence of species-specific odorants and pheromones produced by species-specific biosynthetic mechanisms in mammalian species.
KeywordsProximal Tubular Cell Scent Mark Intact Male Excretion Level Castrate Male
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