Isolation and characterization of a felinine-containing peptide from the blood of the domestic cat (Felis catus).

Felinine is a unique sulfur-containing amino acid found in the urine of domestic cats and select members of the Felidae family. Research over the past 50 years has led to the conclusion that felinine must be synthesized in the kidney, as free felinine is not present in the blood or tissues of cats. We propose that felinine is present in the blood as gamma-glutamylfelinylglycine, a glutathione conjugate. To test our hypothesis [35S]cysteine was administered intraperitoneally to one entire male cat, and two radiolabeled fractions were isolated from the blood. We showed that the amounts of both fractions in serum were linked to the gender of the cat, with entire males expressing significantly higher levels compared with castrated males, entire females, or spayed females. Both fractions were characterized using amino acid analysis with one fraction (S18), containing an equimolar ratio of Cys, Glu, and Gly, while fraction S16 was found to contain Cys, plus free amino acids. Nanospray mass spectrometry confirmed the sequence of fraction S18 as being gamma-glutamylfelinylglycine and conclusively proving that felinine is present in the blood of cats as part of a larger molecule, thereby questioning the current theory that felinine is synthesized in the kidney.