Importance of sulfate, cysteine and methionine as precursors to felinine synthesis by domestic cats ( Felis catus)

There is conflicting evidence in the literature on the utilization of cysteine and methionine as precursors to the urinary sulfur-containing amino acid felinine in cats. Three entire domestic short-haired male cats, housed individually in metabolism cages, were injected intraperitoneally with either [ 35S]-sulfate, [ 35S]-cysteine, or [ 35S]-methionine. Daily urine samples were collected quantitatively for up to 9 days after injection. Each cat was injected once with each compound after observing an appropriate interval for [ 35S] to be depleted between injections. All the urine samples were analysed for felinine content and total radioactivity. Felinine was isolated from each urine sample and analysed for radioactivity. No radioactivity was found in felinine from cats injected with [ 35S]-sulfate. The mean (±S.E.M.) cumulative recovery of radioactivity in the urine of the [ 35S]-sulfate injected cats was 90.6±6.1% after 4 days. The mean (±S.E.M.) cumulative incorporation rate of radioactivity into felinine by the cats receiving the [ 35S]-cysteine and [ 35S]-methionine were 11.6±1.6 and 8.6±0.6%, respectively, after 9 days. The mean (±S.E.M.) cumulative recoveries of radioactivity in the urine were 58.1±3.7 and 36.0±8.0%, respectively. Cysteine and methionine, but not sulfate, are precursors to felinine, with cysteine being a more quantitatively important precursor compared to methionine.