Felinine is a branched-chain sulfur amino acid present in the urine of certain Felidae, including domestic cats. The objective of the present study was to determine if additional cystine and/or dietary N would increase felinine and N-acetylfelinine excretion by intact male cats fed a low-protein (LP) diet. Feeding five adult intact male cats an LP diet (18·8 % of metabolisable energy (ME) as protein) v. a high-protein diet (38·6 % of ME as protein) resulted in a trend (P¼ 0·08) for decreased urinary felinine and no change in N-acetylfelinine excretion. In a 23 d study, when the LP diet was supplemented with L-cystine at 9·3 g/kg DM, urinary felinine:creatinine ratio showed a linear two-fold (121 %) increase (P,0·01) from 0·24 (SEM 0·05) to 0·53 (SEM 0·13) after 10 d. Subsequent feeding of the LP diet resulted in a decrease in felinine excretion to base levels. Plasma gglutamylfelinylglycine concentrations were consistent with the excretion of felinine. Supplementation of the LP diet with L-cystine (9·3 g/kg DM), dispensable amino acids and arginine to a second group (n 5) also resulted in a significant (P,0·01) but smaller (þ 72 %) increase in the daily felinine:creatinine ratio (0·25 (SEM 0·04) to 0·43 (SEM 0·05)). The degree of felinine N-acetylation within groups was unaffected by dietary addition and withdrawal of amino acids. The results indicate that felinine synthesis is regulated by cystine availability, and that arginine may be physiologically important in decreasing felinine biosynthesis in intact male cats.
N-acetylfelinine: Arginine: Cats: Cysteine: FelinineFelinine is a unique amino acid excreted by certain species within the Felidae family, including the domestic cat. This sulfur-containing, branched-chain amino acid has been found in concentrations of 3·6 g/l in the urine of intact male cats (Felis catus), with castrated males excreting approximately one-quarter as much as intact males (1) . Synthesis of felinine is believed to occur from glutathione and isopentenylpyrophosphate, through a unique glutathione sulfur transferase enzyme which is under hormonal control (2,3) to form a felinine-containing tripeptide: g-glutamylfelinylglycine or methylbutanolglutathione (MBG). The g-glutamylfelinylglycine is transported in the blood (4) to the kidney where hydrolysis by g-glutamyltransferase present on the surface of the brushborder membrane of proximate tubular cells results in felinylglycine and glutamic acid. The dipeptide can be further hydrolysed to felinine and glycine by either dipeptidase activities or a recently discovered enzyme with carboxylesterase activity (5) . This 70 kDa enzyme (cauxin) has been found in relatively high concentrations in feline urine and has been shown to be able to hydrolyse felinylglycine but not MBG (5) . Following the hydrolysis of felinylglycine, N-acetylation of felinine can occur. This is an intracellular process catalysed by cysteine-S-conjugate N-acetyltransferase and yielding mercapturic acids via the addition of an acetyl group transferred from acetyl-CoA, whi...