LC–MS/MS quantification of felinine metabolites in tissues, fluids, and excretions from the domestic cat (Felis catus)

Domestic cat urine contains large concentrations of the unusual amino acid, felinine (2-amino-7-hydroxy-5,5-dimethyl-4-thiaheptanoic acid). A felinine derivative, 3-mercapt-3-methylbutanol is a potential scent signal for letting other animals know that the scent owners are cats. 3-Methylbutanol-glutathione (MBG) is an upstream precursor of the biosynthetic pathway of felinine that may be produced in hepatocytes by conjugation of glutathione with isopentenyl pyrophosphate, an intermediate for cholesterol biosynthesis. However, little evidence exists to support the biosynthesis of MBG in the liver. This study examined the distribution of metabolites of the felinine biosynthetic pathway in multiple tissues, body fluids, and excretions of cats. MBG, the felinine precursor, 3-methylbutanol-cysteinylglycine (MBCG), felinine, and felinine N-acetyl derivative were quantified by liquid chromatography-electron spray ionization-tandem mass spectrometry. All compounds were detected in cat serum. In males, MBG and MBCG contents were significantly higher than felinine and N-acetylfelinine contents. MBG was detected in multiple tissues, including the salivary gland, heart, liver, spleen, gut, kidney, bladder, adipose tissue, and muscle. Sex differences in MBG levels were observed in the liver and other tissues. Felinine and N-acetylfelinine were also detected in those tissues. Furthermore, we detected all compounds in cat bile and fecal samples, indicating that felinine is excreted into the feces via bile from the liver. We conclude that MBG is synthesized in several tissues in a sex-dependent manner. These findings improve our understanding of felinine metabolism and function in cats.