Relationship between plasma testosterone and urinary felinine in the growing kitten

Minimum methionine requirement in adult cats as determined by indicator amino acid oxidation

There is a lack of empirical data on the dietary Met requirement, in the presence of Cys or cystine, in adult cats. Thus, the aim of this study was to determine the Met requirement, in the presence of excess Cys, in adult cats at maintenance using the indicator amino acid oxidation (IAAO) technique. Six adult neutered male cats were initially selected and started the study. Cats were adapted to the basal diet sufficient in Met (0.24% dry matter, DM) for 14 d prior to being randomly allocated to one of eight dietary levels of Met (0.10%, 0.13%, 0.17%, 0.22%, 0.27%, 0.33%, 0.38%, and 0.43% DM). Different dietary Met concentrations were achieved by supplementing the basal diet with Met solutions. Alanine was additionally included in the solutions to produce isonitrogenous and isoenergetic diets. Cats underwent a 2-d adaptation period to each experimental diet prior to each IAAO study day. On IAAO study days, 13 meals were offered corresponding to 75% of each cat's daily food allowance. The remaining 25% of their daily food intake was offered after each IAAO study. A bolus dose of NaH13CO3 (0.44 mg kg-1) and l-[1-13C]-phenylalanine (13C-Phe; 4.8 mg kg-1) were provided in fifth and sixth meals, respectively, followed by a constant dose of 13C-Phe (1.04 mg kg-1) in the next meals. Breath samples were collected and total production of 13CO2 was measured every 25 min through respiration calorimetry chambers. Steady state of 13CO2 achieved over at least three breath collections was used to calculate oxidation of 13C-Phe (F13CO2). Competing models were applied using the NLMIXED procedure in SAS to determine the effects of dietary Met on 13CO2. Two cats were removed from the study as they did not eat all meals, which is required to achieve isotopic steady. A breakpoint for the mean Met requirement, with excess of Cys, was identified at 0.24% DM (22.63 mg kg-1) with an upper 95% confidence limit of 0.40% DM (37.71 mg·kg-1), on an energy density of 4,164 kcal of metabolizable energy/kg DM calculated using the modified Atwater factors. The estimated Met requirement, in the presence of excess of Cys, is higher than the current recommendations proposed by the National Research Council's Nutrient Requirement of Dogs and Cats, the Association of American Feed Control Officials, and the European Pet Food Industry Federation.

Sociality of Cats toward Humans Can Be Influenced by Hormonal and Socio-Environmental Factors: Pilot Study

Individual differences in the sociality of cats are influenced by inherited and environmental factors. We recently revealed that hormones can make a difference in intraspecies social behavior. It remains unclear whether cat behavior toward humans is modulated by hormones. Therefore, we analyzed the relationship between cat behavior and their basal hormone concentrations after spending time together with human experimenters. In addition, we analyzed the relationship between cat behavior and the timing of when the individual cats began living with a human because the sociality of cats could be dependent on their developmental experiences. The results showed that male cats that began living with humans earlier had more contact with an experimenter. In addition, individual male cats with low testosterone levels were more likely to interact with an experimenter. These findings of this pilot study suggest that the sociality of male cats toward humans is affected by testosterone and the age at which they begin to live with humans.

Control of felinine-derived malodor in cat litter

OBJECTIVES

Malodors stemming from soiled cat litter are a major frustration for cat owners, despite the widespread use of absorbent litters with claims of odor control. Technologies for effective litter odor control have not been rigorously evaluated. Here, we report on the effectiveness of a novel litter formulation of 1-monochlorodimethylhydantoin (MCDMH)-modified clinoptilolite zeolite (MCDMH-Z) to control the odors of 3-mercapto-3-methylbutanol (3M3MB) and ammonia, the principal products generated by the enzymatic breakdown of felinine and urea, respectively.

METHODS

The efficacy of MCDMH-Z for the odor control of 3M3MB was determined by solid-phase microextraction and gas chromatography mass spectrometry analysis, colorimetric analysis and a sensory panel. Enzyme inhibition was monitored by a colorimetric coupled assay for ammonia. The antimicrobial properties were measured by a reduction in colony-forming units (CFUs).

RESULTS

3M3MB proved highly susceptible to modification by MCDMH-Z granules. Headspace above litter exposed to MCDMH-Z showed no detectable 3M3MB; levels >59 ng were detected in commercially available products. Urease activity decreased by >97% after incubation with MCDMH-Z to 0.14 mg/ml. Cat litter F showed comparable inhibition (0.13 mg/ml); others showed less inhibition, producing up to 4.8 mg/ml of ammonia. MCDMH-Z reduced the CFUs of Proteus vulgaris by six log reduction values in 30 mins; in the same amount of time, no reduction was seen with commercial products tested. The odor control capability of the MCDMH-Z granules was further supported by a sensory panel scoring 3M3MB-spiked litters.

CONCLUSIONS AND RELEVANCE

Samples of commercially available litter products showed an effect on malodor, or inhibition of urease, or contained antimicrobial activity; no samples were capable of accomplishing these concurrently. In contrast, MCDMH-Z granules were effective in all three test categories. Control of felinine-derived odors, in particular, has the potential to improve cat owner satisfaction, and may beneficially affect cat behaviors provoked by pheromonally active sulfurous metabolites deposited in the litter.

Neutering Effects on Social Behaviour of Urban Unowned Free-Roaming Domestic Cats

The "trap, spay/neuter, and release" programs to manage unowned free-roaming cat populations are diffused worldwide and they are largely advised even in countries where the suppression of unowned cats is enforced by law. Despite the massive neutering campaigns in the world, there is little information on the influence of neutering on individual cat behaviour, as well as on the social structure of cat colonies. The aim of this study is to verify such effects. Before neutering, the group consisted of 17 free-roaming domestic cats, who were totally unrestrained. After neutering it consisted of 16 individuals. Data on the outcomes of aggressive, submissive, affiliative, and territorial behaviour were collected, using classic ethological methods ("focal animal", "all occurrences", and "1/0" sampling methods) (735 h of observation before and 537 h after neutering). The dominance hierarchy did not change after neutering. On the other hand, the frequency of aggressive, territorial behaviour as well as proximity among individuals decreased significantly. Affiliative behaviour has been observed among neutered adult males that never performed it before neutering. The results of this study suggest that, after neutering: (i) The group of free-ranging cats was stable over time; (ii) the hierarchical structure of the cat social group did not change; (iii) the general level of activity, decreased; (iv) urine spraying marking behaviour almost disappeared; (v) the level of social proximity among group members decreased, although it tended to increase in some male-male dyads. The most obvious effect of neutering, detected on individual behaviour, was that cats were less active.

The Views of the UK Public Towards Routine Neutering of Dogs and Cats

Despite being routinely recommended by veterinarians, neutering of dogs and cats has both positive and negative impacts on animal welfare and is ethically problematic. We examined attitudes of a sample of the UK public towards routine neutering of dogs and cats using a questionnaire. Respondents indicated their level of agreement with statements describing welfare and ethical reasons 'for' and 'against' the neutering of male and female dogs and cats. We conducted a general linear model (GLM) analysis to investigate the effects of demographic factors on agreement scores. Respondents (n = 451) expressed views both supporting and opposing neutering. The predominant view (>80%) supported neutering, justified primarily by prevention of unwanted offspring and reproductive diseases. Around 10% of the respondents disagreed and felt that neutering should only be done for medical reasons. Men were less likely than women to support neutering (p < 0.001). Those with meat reduction diets were more likely to be against neutering (p < 0.05) and cat owners supported neutering more than non-cat owners (p < 0.05). Although the data reflected a wide range of ethical views, our findings show that the UK public generally supports the routine neutering of dogs and cats. This insight has implications for future policy-making and compliance with veterinary advice.

Assessing puberty in ex situ male cheetahs (Acinonyx jubatus) via fecal hormone metabolites and body weights

Cheetahs are one of the most heavily studied felid species, with numerous publications on health, disease, and reproductive physiology produced over the last 30 years. Despite this relatively long history of research, there is a paucity of crucial biological data, such as pubertal onset, which has direct and significant applications to improved management of ex situ cheetah populations. This study aimed to determine age of pubertal onset in ex situ male cheetahs using non-invasive fecal steroid hormone monitoring and body weights. Fecal samples from 12 male cheetahs from four institutions were collected 2-3 times weekly from 1 to 42 months of age. Fecal androgen and glucocorticoid metabolites were analyzed using enzyme immunoassays previously validated for use with cheetah feces. Animal body weights were recorded monthly. Fecal hormone and body weight data were analyzed using generalized linear mixed models. Androgen concentrations exhibited an increase to levels similar to those observed in adult males by 18-24 months of age, and males attained adult body weights by 21 months of age. Based on these weight data and the initial increase in androgens toward adult concentrations, males were considered pubertal from 18 to 24 months of age. Glucocorticoid concentrations and amplitude of concentration over baseline were also increased during this period. Knowledge about the physiological changes associated with puberty is useful for management and improving reproductive success of cheetah populations under human care, particularly for determining timing of litter separation from dam, littermate dispersal and when to introduce potential breeding pairs.

Metabolic Profiling Reveals Effects of Age, Sexual Development and Neutering in Plasma of Young Male Cats

Neutering is a significant risk factor for obesity in cats. The mechanisms that promote neuter-associated weight gain are not well understood but following neutering, acute changes in energy expenditure and energy consumption have been observed. Metabolic profiling (GC-MS and UHPLC-MS-MS) was used in a longitudinal study to identify changes associated with age, sexual development and neutering in male cats fed a nutritionally-complete dry diet to maintain an ideal body condition score. At eight time points, between 19 and 52 weeks of age, fasted blood samples were taken from kittens neutered at either 19 weeks of age (Early Neuter (EN), n = 8) or at 31 weeks of age (Conventional Neuter (CN), n = 7). Univariate and multivariate analyses were used to compare plasma metabolites (n = 370) from EN and CN cats. Age was the primary driver of variance in the plasma metabolome, including a developmental change independent of neuter group between 19 and 21 weeks in lysolipids and fatty acid amides. Changes associated with sexual development and its subsequent loss were also observed, with differences at some time points observed between EN and CN cats for 45 metabolites (FDR p<0.05). Pathway Enrichment Analysis also identified significant effects in 20 pathways, dominated by amino acid, sterol and fatty acid metabolism. Most changes were interpretable within the context of male sexual development, and changed following neutering in the CN group. Felinine metabolism in CN cats was the most significantly altered pathway, increasing during sexual development and decreasing acutely following neutering. Felinine is a testosterone-regulated, felid-specific glutathione derivative secreted in urine. Alterations in tryptophan, histidine and tocopherol metabolism observed in peripubertal cats may be to support physiological functions of glutathione following diversion of S-amino acids for urinary felinine secretion.

Spontaneous hormonal variations in male cats following gonadectomy

The increased prevalence of obesity after neutering in cats is problematic in veterinary practice. Although many factors seem to be involved, the role of prolactin (PRL) and insulin-like growth factor-I (IGF-I), both implicated in adipose tissue development and glucose intolerance, should be considered. Seven male cats were castrated when 11 months old. Body weight was then recorded for 56 weeks and PRL, IGF-I and leptin assayed for 44 weeks. Body weight increased steadily but only significantly after 36 weeks. It stabilised after 44 weeks, and the cats then gained about 20% of their initial body weight. IGF-I increased rapidly and was significantly higher by week 3. PRL and leptin increased with initial peaks during the eighth and eleventh weeks, respectively. This study confirms that castration rapidly modifies the hormonal balance, partly explaining the body weight increase, and that hormonal changes precede this body weight increase. Hyperleptinaemia is apparently a consequence of excess weight.

Peculiarities of one-carbon metabolism in the strict carnivorous cat and the role in feline hepatic lipidosis

Research in various species has indicated that diets deficient in labile methyl groups (methionine, choline, betaine, folate) produce fatty liver and links to steatosis and metabolic syndrome, but also provides evidence of the importance of labile methyl group balance to maintain normal liver function. Cats, being obligate carnivores, rely on nutrients in animal tissues and have, due to evolutionary pressure, developed several physiological and metabolic adaptations, including a number of peculiarities in protein and fat metabolism. This has led to specific and unique nutritional requirements. Adult cats require more dietary protein than omnivorous species, maintain a consistently high rate of protein oxidation and gluconeogenesis and are unable to adapt to reduced protein intake. Furthermore, cats have a higher requirement for essential amino acids and essential fatty acids. Hastened use coupled with an inability to conserve certain amino acids, including methionine, cysteine, taurine and arginine, necessitates a higher dietary intake for cats compared to most other species. Cats also seemingly require higher amounts of several B-vitamins compared to other species and are predisposed to depletion during prolonged inappetance. This carnivorous uniqueness makes cats more susceptible to hepatic lipidosis.

Felinine excretion in domestic cat breeds: a preliminary investigation

The aim of this study was to determine possible differences in felinine excretion between domesticated cat breeds. For this purpose, urine was collected from a total of 83 privately owned entire male cats from eight different breeds in the Netherlands during the period of November 2010 till November 2011. In the collected samples, free felinine and creatinine concentrations were measured. Free felinine concentrations were expressed relative to the urinary creatinine concentration to compensate for possible variations in renal output. The mean (±SD) felinine:creatinine (Fel:Cr) ratio as measured over all cats was 0.702 (±0.265). Both the Abyssinian and Sphynx breeds showed the highest Fel:Cr ratio (0.878 ± 0.162 and 0.878 ± 0.341 respectively) which significantly differed from the ratios of the British Shorthairs (0.584 ± 0.220), Birmans (0.614 ± 0.266), Norwegian Forest cats (0.566 ± 0.296) and Siberian cats (0.627 ± 0.124). The Fel:Cr ratios of the Persians (0.792 ± 0.284) and Ragdolls (0.673 ± 0.256) showed no statistical difference with either of the other breeds. A significant proportion of the observed variation between the different feline breeds could be explained by hair growth, as both hair growth and felinine production compete for available cysteine. Shorthaired and hairless cat breeds generally showed a higher Fel:Cr ratio compared to longhaired cat breeds, with the exception of Persian cats. Further research is warranted to more closely study the effect of hair growth on felinine production.

Accelerated evolution of CES7, a gene encoding a novel major urinary protein in the cat family

Cauxin is a novel urinary protein recently identified in the domestic cat that regulates the excretion of felinine, a pheromone precursor involved in sociochemical communication and territorial marking of domestic and wild felids. Understanding the evolutionary history of cauxin may therefore illuminate molecular adaptations involved in the evolution of pheromone-based communication, recognition, and mate selection in wild animals. We sequenced the gene encoding cauxin, CES7, in 22 species representing all major felid lineages, and multiple outgroups and showed that it has undergone rapid evolutionary change preceding and during the diversification of the cat family. A comparison between feline cauxin and orthologous carboxylesterases from other mammalian lineages revealed evidence of strong positive Darwinian selection within and between several cat lineages, enriched at functionally important sites of the protein. The higher rate of radical amino acid replacements in small felids, coupled with the lack of felinine and extremely low levels of cauxin in the urine of the great cats (Panthera), correlates with functional divergence of this gene in Panthera, and its putative loss in the snow leopard. Expression studies found evidence for several alternatively spliced transcripts in testis and brain, suggesting additional roles in male reproductive fitness and behavior. Our work presents the first report of strong positive natural selection acting on a major urinary protein of nonrodent mammals, providing evidence for parallel selection pressure on the regulation of pheromones in different mammalian lineages, despite the use of different metabolic pathways. Our results imply that natural selection may drive rapid changes in the regulation of pheromones in urine among the different cat species, which in turn may influence social behavior, such as territorial marking and conspecific recognition, therefore serving as an important mechanism for the radiation of this group of mammals.

Testosterone increases urinary free felinine, N-acetylfelinine and methylbutanolglutathione excretion in cats (Felis catus)

Two days after castration, urinary free felinine plus N-acetylfelinine decreased 24% in male cats, but, by day 5, the concentration had not decreased to that routinely found in males that have been castrated for several months. In a second experiment, three groups of castrated adult male cats received different subcutaneous injections: control (carrier), testosterone, testosterone plus estradiol. A fourth group of intact adult female cats received a testosterone injection. Urine was collected and analysed for free felinine, N-acetylfelinine and 3-methylbutanolglutathione. Baseline blood testosterone and estradiol concentrations were low during the pre-period, but increased sharply after hormone injections. The concentration of all three urinary metabolites increased as a result of testosterone injections with estradiol not modulating the effect. The effect of testosterone was not gender dependent. The concentration of free felinine, N-acetylfelinine and 3-methylbutanolglutathione in the urine remained low in the placebo control group throughout the study. The relative molar contribution of free felinine to the total amount of felinine containing compounds increased due to testosterone treatment, while the contribution of 3-methylbutanolglutathione and N-acetylfelinine decreased. Testosterone increases free felinine, N-acetylfelinine and 3-methylbutanolglutathione excretion in castrated adult male and intact female cats, whereas estradiol does not modulate this effect.

Characterization of cauxin in the urine of domestic and big cats

Cauxin is an abundant protein in feline urine. We have used proteomics strategies to characterize cauxin from the urine of domestic cats and a number of big cat species. Proteins were resolved by gel-based electrophoretic purification and subjected to in-gel digestion with trypsin. The resultant tryptic peptides were mass-measured by matrix-assisted laser desorption ionization time of flight mass spectrometry. Peptides were also resolved by liquid chromatography and analyzed by electrospray ionization and tandem mass spectrometry to generate fragment ion data to infer the amino acid sequence. We identified cauxin polymorphisms and corrected a sequencing artifact in cauxin from the domestic cat. The proteomics data also provided positive evidence for the presence of a cauxin homolog in the urine of big cats (Pantherinae), including the Sumatran tiger, Asiatic lion, clouded leopard, Persian leopard, and jaguar. The levels of cauxin in the urine of all big cats were substantially lower than that in the urine of intact male domestic cats.

Application of Metabonomics in a Comparative Profiling Study Reveals N-Acetylfelinine Excretion as a Biomarker for Inhibition of the Farnesyl Pathway by Bisphosphonates

In this work, the results of metabolic profiling of urine from a preclinical comparative profiling study with the two biphosphonates ibandronate and zoledronate are reported. Toxicological assessment showed very different effects for the two compounds. Ibandronate did not cause major signs of toxicity, whereas zoledronate elicited hepatotoxicity and nephrotoxicity. Increased levels of urinary glucose and decreased levels of urinary creatinine detected by NMR also indicated drug-induced nephrotoxicity. Similarly, increased urinary levels of creatine and taurine indicated hepatotoxicity. Both organ toxicities were later confirmed by histopathology. In addition, the benefit of metabonomics as an open approach as compared to targeted methods was demonstrated by the identification of an unknown molecule in the urine of rats dosed with zoledronate. The structure elucidation revealed this molecule as N-acetylfelinine. Analysis of the pathways proposed for the biochemical synthesis of this molecule showed that the synthesis and excretion of N-acetylfelinine could easily be explained by drug-induced inhibition of farnesyl diphosphate synthase. This is the reported mode of action of bisphosphonates. Until now, N-acetylfelinine was exclusively observed in the urine of felidae species, where it is believed to be a precursor to a pheromone.

A Major Urinary Protein of the Domestic Cat Regulates the Production of Felinine, a Putative Pheromone Precursor

Domestic cats spray urine with species-specific odor for territorial marking. Felinine (2-amino-7-hydroxy-5,5-dimethyl-4-thiaheptanoic acid), a putative pheromone precursor, is excreted in cat urine. Here, we report that cauxin, a carboxylesterase excreted as a major urinary component, regulates felinine production. In vitro enzyme assays indicated that cauxin hydrolyzed the felinine precursor 3-methylbutanol-cysteinylglycine to felinine and glycine. Cauxin and felinine were excreted age dependently after 3 months of age. The age-dependent increases in cauxin and felinine excretion were significantly correlated. In mature cats, cauxin and felinine levels were sex-dependently correlated and were higher in males than in females. In headspace gas of cat urine, 3-mercapto-3-methyl-1-butanol, 3-mercapto-3-methylbutyl formate, 3-methyl-3-methylthio-1-butanol, and 3-methyl-3-(2-methyldisulfanyl)-1-butanol were identified as candidates for felinine derivatives. These findings demonstrate that cauxin-dependent felinine production is a cat-specific metabolic pathway, and they provide information for the biosynthetic mechanisms of species-specific molecules in mammals.

Species-, sex-, and age-dependent urinary excretion of cauxin, a mammalian carboxylesterase

Domestic cats exhibit physiological proteinuria due to the excretion of cauxin, a carboxylesterase, into the urine. In the present report, we demonstrate that cauxin is excreted in a species-, sex-, and age-dependent manner. Although the cauxin gene is conserved in mammals, including human, mouse, and dog, urinary cauxin was found only in member of the genus Felis and lynx (bobcat, and lynx) and not in other Felidae (genus: Panthera and puma) tested. In mature cats, cauxin excretion was higher in intact males than in castrated males or in intact or spayed females. Daily cauxin excretion decreased immediately after castration. Immunohistochemistry confirmed that cauxin expression in the kidney proximal straight tubules was higher in intact males than in castrated males. Urinary cauxin was detectable by Western blotting in cats older than about 3 months, and its excretion increased with age. In a zymographic esterase assay, urine contained a major cauxin band; by contrast, kidney homogenates contained three major bands, comprising two carboxylesterases and an unidentified esterase, and one minor cauxin band. These results suggest that 1. cauxin excretion is regulated by sex hormones, such as testosterone, 2. cauxin functions as an esterase in the urine rather than in kidney cells, and 3. the decomposition products by cauxin are excreted in a species-, sex-, and age-dependent manner, as is cauxin itself.

Assessment of the immunological and biological efficacy of two different doses of a recombinant GnRH vaccine in domestic male and female cats (Felis catus)

This study has assessed the immunological and biological efficacy of two different doses of a recombinant GnRH vaccine administered to intact domestic cats. Fifteen kittens, 8-9 weeks of age, were allocated to three unequal treatment groups: group 1, 1 male control cat; group 2, 5 females administered 10 microg of GnRH antigen; and group 3, 4 males and 5 females administered 100 microg of GnRH antigen. Animals in groups 2 and 3 were immunized at study days 0, 28 and 643. One of the four males (AJZ3) in the high dose group showed a more rapid decline in GnRH antibody titers and received an additional immunization at day 461. Blood samples were collected on study days 28, 35, 56, 97, 157, 213, 270, 325, 377, 433, 496, 549, 605, and 685. The injection sites were monitored for tissue reactivity on study days 5, 7, 12, and 28. The animals' general health and demeanor was monitored on a daily basis. Sera obtained from 11 animals on day 549 were submitted for biochemistry analysis. Two males and two females were necropsied at the completion of the study and histopathological examination of the gonads, hypothalamus, pituitary, kidneys and uterus was performed. All 14 immunized animals developed immunoneutralizing titers to GnRH. GnRH titers peaked at day 56 and 13 of 14 cats maintained these titers for >20 months. Except for AJZ3, the immunized males' serum testosterone concentrations were below the assay's level of detection after the second immunization. None of the 10 immunized females showed signs of estrous behavior or became pregnant. Testicular and ovarian histology was consistent with suppression of LH and FSH activity. The majority of tissue reactions resolved by 28 days post-vaccination. Serum biochemistry and tissue histopathology revealed no evidence of tissue or organ damage. This study was unique in that a recombinant GnRH antigen was used to stimulate and maintain biologically relevant titers in very young male and female cats for at least 20 months.

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.

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.