The Journal of Nutrition Vol. 128 No. 4 April 1998, pp. 751-757

Taurine Status in Cats Is Not Maintained by Dietary Cysteinesulfinic Acid^1,2

Susan E. Edgar³, Claudia A. Kirk⁴, Quinton R. Rogers, and James G. Morris⁵

Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA 95616-8741

	ABSTRACT

Abstract Introduction Methods Results Discussion References

Endogenous synthesis of taurine by cats is limited. Putative precursors of taurine, cysteinesulfinic acid and cysteic acid, were fed to cats to determine whether they were utilized. Groups of five cats were depleted of taurine by a resin (Colestipol®) diet, then given 6 dietary treatments containing (g/kg diet): 0.0, 0.4, or 0.8 taurine; or 0.98 or 1.96 cysteinesulfinic acid, or 0.4 taurine + 1.0 cysteic acid for 12 wk. Plasma and whole blood taurine concentrations and body weights were measured weekly. Concentration of taurine in semitendinosus muscle was measured initially, after 2 wk of taurine depletion (after resin-diet), and monthly thereafter. The resin diet decreased concentrations of taurine in plasma, whole blood, and muscle to 0.20, 0.49, and 0.37 of initial values, respectively. Cysteinesulfinic acid diets resulted in no significant (P > 0.05) increase in the concentration of taurine in plasma, whole blood, or muscle, and no increased excretion of cysteinesulfinate or taurine in urine or feces. Cats fed the diets containing 1.0 g cysteic acid + 0.4 g taurine, or 0.8 g taurine/kg diet had similar concentrations of taurine in plasma, whole blood, and muscle. Aminotransferase activity for cysteinesulfinic acid in the liver and intestinal mucosa of cats and rats was higher than that for aspartic or cysteic acids. Transamination of dietary cysteinesulfinic acid to -sulfinylpyruvate (which spontaneously decomposes), rather than decarboxylation is postulated as the basis for no detectable conversion to taurine. In contrast, cysteic acid is reversibly transaminated to -sulfopyruvate which is stable and thereby is a precursor for taurine in cats.

	INTRODUCTION

Abstract Introduction Methods Results Discussion References

Taurine is a dietary essential sulfur amino acid for cats. Previously, we demonstrated that dietary cysteic acid (CA)⁶ could serve as a precursor of taurine in cats (Edgar et al. 1994). Studies in rats have shown that cysteinesulfinic acid is an intermediate in the catabolic pathway of cysteine (Jacobsen et al. 1968) and is a precursor in the biosynthesis of taurine (Singer and Kearney 1954). In rats, degradation of cysteine occurs primarily via oxidation of cysteine to cysteinesulfinic acid, a reaction catalyzed by cysteine dioxygenase (CD) (Ewetz and Sörbo 1966, Griffith 1983, Weinstein et al. 1988). Cysteinesulfinic acid may either be transaminated to -sulfinylpyruvate, which nonenzymatically yields pyruvate and sulfite (Singer and Kearney 1956) or alternatively, decarboxylated by the enzyme cysteinesulfinic acid decarboxylase (CSAD) to hypotaurine, which is then further oxidized to taurine (Kontro and Oja 1980).

Endogenous taurine biosynthesis is limited in cats (Rentschler et al. 1986). Measurement of enzyme activities in the degradative pathway of cysteine has led many investigators to suggest that the difference in activity of CSAD among species may account for the observed difference in taurine synthetic rate (De La Rosa and Stipanuk 1985, Jacobsen and Smith 1968, Knopf et al. 1978, Stipanuk and Rotter 1984). It has been suggested that CD may limit taurine synthesis in cats (Park 1991) which is in agreement with an earlier report of Knopf et al. (1978) that showed low CD activity in the liver of cats. In a labeling study, Hardison et al. (1977) added [¹⁴C]-cysteinesulfinic acid to a liver perfusate solution and determined that the hepatic taurine synthesis rate from cysteinesulfinic acid was about twice as rapid in rat as in cat liver. Human liver is more similar to cat than rat liver in its taurine synthetic capacity, despite cats having a higher activity of CSAD (Jacobsen and Smith 1968). Although both cats and humans synthesize taurine, taurine is not essential in the diet of adult humans (Blaschko et al. 1953, Hope 1953, Jacobsen and Smith 1968), presumably because humans can conserve taurine by conjugating cholic acid with glycine when dietary taurine is low, and possibly due to differences in synthetic capacity.

In a study by Stipanuk and Rotter (1984), the route of administration of L-cysteinesulfinic acid affected its metabolic fate in rats. When L-[1-¹⁴C] and L-[3-¹⁴C] cysteinesulfinic acid were given by i.p. injection, 70% was recovered as taurine over an 8 h period. When the same radiolabeled L-cysteinesulfinic acid was given via gastric intubation, no labeled taurine was recovered. De La Rosa et al. (1985) demonstrated a lower activity of CSAD in young cats as compared to young and adult rats, thus, it has been repeatedly suggested that the low efficiency of conversion of cysteine to taurine in cats is the result of a total of low CSAD activity. Nevertheless, it is our assertion that in cats the limited biosynthesis of taurine is the result of the very low CD (Park 1991). Thus, consistent with the work of Stipanuk and co-workers with rats, we postulated that transamination of cysteinesulfinic acid within the intestine was virtually complete, preempting the synthesis of taurine from dietary cysteinesulfinic acid by the liver or other tissues.

The addition of cysteic acid to diets containing inadequate taurine improves taurine status of cats. In vivo studies have indicated that cysteic acid can be decarboxylated to form taurine in rats (Singer 1975, Weinstein et al. 1988). The decarboxylase of L-cysteinesulfinic acid is reported to be the same as that for cysteic acid (Blaschko et al. 1953), thus it would be predicted that cats decarboxylate some cysteic acid to form taurine. Despite the reported low CSAD activity in cats (Blaschko et al. 1953, Davison 1956, Guion-Rain et al. 1972, Hope 1955, Jacobsen et al. 1964, Sörbo et al. 1957), two studies have demonstrated the in vivo conversion of dietary cysteic acid to taurine when it is added to either a commercial canned diet (Earle and Smith 1992), or to a purified diet (Edgar et al. 1994). Our study was designed to determine whether dietary cysteinesulfinic acid could provide the taurine needs of cats, and if not, whether intestinal and hepatic aspartate aminotransferase activity might explain the lack of utilization of cysteinesulfinic acid compared to cysteic acid for taurine synthesis in cats.

	MATERIALS AND METHODS

Abstract Introduction Methods Results Discussion References

Animals and diets.  Thirty adult specific pathogen-free cats from the Nutrition and Pet Care Center at the University of California-Davis, with an initial body weight of 3543 ± 613 g (mean ± SEM) were used. For >6 mo before entering the 14-wk study, cats received a nutritionally complete commercial expanded diet containing ~ 1.2 g taurine/kg diet. For a 1 wk adaptation period, cats were fed a purified diet containing 1.5 g taurine/kg diet (Table 1); then, they were given a taurine-free diet (resin diet) containing Colestipol® HCl (Upjohn, Kalamazoo, MI) 30 g/kg diet, which was fed for 2 wk to deplete the body stores of taurine. Cats were grouped on the basis of body weight, (thus, no group contained exclusively males or females) then cats within groups were randomly assigned to the 6 dietary treatments. Four of the dietary treatments contained 2 female and 3 male cats, whereas the other two dietary treatments had 3 female and 2 male cats. The experimental diets contained taurine (TAU), L-cysteinesulfinic acid (CSA), or L-cysteic acid (CA)⁷ as follows (g/kg diet): 0 TAU-0 CSA-0 CA (negative control); 0.4 or 0.8 TAU (positive controls); 0 TAU-0.98 CSA, 0 TAU-1.96 CSA, 0.4 TAU-1.0 CA. The amount of dietary L-cysteinesulfinic acid used was isomolar to the quantity of L-cysteic acid fed in previous work (Edgar et al. 1994). The 0.4 TAU-1.0 CA group was included to examine whether cysteic acid would support normal taurine status in cats marginally deficient in taurine, since this treatment group was not included in the previous study (Edgar et al. 1994). Cats were given these diets for 12 wk with free access to food and water. Cats were housed individually in stainless steel metabolism cages fitted with trays for quantitative collection of feces and urine, in a room maintained at 24-26°C with a 14:10 h light:dark cycle. To prevent microbial growth, urine was maintained at pH<2 by acidification with HCl, 6 mol/L. Daily urine samples were collected separately from daily fecal collections. The experimental protocol was approved by University of California-Davis Animal Use and Care Administrative Advisory Committee and was carried out in accordance with standards of the Guide for the Care and Use of Laboratory Animals (1985) of the NIH.

Procedures.  Daily food intakes and weekly body weights were recorded. Each week, between 1000 and 1200 h, a blood sample was collected from the jugular vein of unanesthetized cats into heparinized 3 mL syringes for measurement of whole blood and plasma taurine concentrations. Plasma was separated by centrifugation at 3000 × g for 15 min, excluding the buffy layer. Biopsy samples of the right semitendinosus muscle were obtained initially, at 3 wk (post-resin diet), and monthly while the cats consumed the experimental diets (wk 7, 11, and 15). Cats were anesthetized (mixture of ketamine and valium), and biopsy samples (14-63 mg wet weight) of the semitendinosus muscle were excised using a 14-gauge, Tru-cut® biopsy needle (Travenol Laboratories, Deerfield, IL). Urine and feces were collected at daily intervals for 7 d, at wk 1 (initial), wk 3 (after feeding the resin diet for 2 wk), and wk 9 and 15 (wk 6 and 12 of dietary treatments). Pooled composite samples were stored at 20°C until analyzed for taurine and cysteinesulfinic acid. The proportion of bound taurine was calculated for urine and fecal samples as the difference between taurine concentrations before and after acid hydrolysis.

Amino acid analyses.  Taurine concentrations in plasma, whole blood, semitendinosus muscle, urine, and feces were measured using an automated amino acid analyzer (Beckman Model 121-MB, Beckman Instruments, Palo Alto, CA 94304). Urinary and fecal concentrations of cysteinesulfinate were analyzed by HPLC using a Millipore Waters HPLC system, Model 745B (Waters, Division of Millipore, Milford, MA) following the methods of Kuriyama and Tanaka (1984) and Stipanuk et al. (1987). For measurement of total taurine and cysteinesulfinate, 2.5 mL of urine or 0.5 g of feces were hydrolyzed for 24 h at 100°C with an equal amount of concentrated HCl, (12 mol/L), evaporated to dryness, then reconstituted with 1.0 mL lithium citrate diluting buffer (pH 2.2, Beckman Instruments, Inc., Palo Alto, CA) before being passed through a mixed-bed ion-exchange column. Polystyrene columns (diameter = 0.7 cm) with plastic filter discs (Isolab Inc., Akron, OH) were prepared by layering approximately 1 cm of AG 1X8 100/200 mesh resin in the chloride form over 1 cm of AG 50WX8 200/400 mesh resin in the hydrogen form. The column was washed with 20 mL of distilled water, a 0.2 mL sample from urine or feces (adjusted to a pH between 5 and 10) was added to the column and eluted with 1.8 mL of distilled water, allowing the resin bed to completely drain. The eluted sample was buffered by the addition of 0.1 to 0.4 mL of a lithium citrate buffer at pH 2.2 (Beckman Instruments, Palo Alto, CA) and analyzed using an amino acid analyzer for taurine or HPLC for cysteinesulfinate. Mean recoveries of taurine and cysteinesulfinic acid from the mixed-bed ion-exchange column was 92.0 ± 7.2 and 98.8 ± 4.7%, respectively.

View larger version (24K): [in this window] [in a new window]   Fig 1. Concentrations of taurine in the plasma of cats given diets containing either taurine (TAU), cysteinesulfinic acid (CSA) or cysteic acid (CA). Values are means ± SEM, n = 5.

Aminotransferase assay.  The principle of this assay is a quantitative conversion of cysteinesulfinic acid to pyruvate, via spontaneous breakdown of -sulfinylpyruvate, or cysteic acid to -sulfopyruvate, respectively. The pyruvate formed is reduced to lactate by lactate dehydrogenase, coupled to the equivalent oxidation of NADH to NAD. The -sulfopyruvate (stable) is reduced by malate dehydrogenase coupled to the quantitative oxidation of NADH to NAD (Weinstein and Griffin 1988). The enzymatic transamination of L-aspartate, L-cysteinesulfinate or L-cysteate was diluted 1:50, then assayed by spectrophotometric measurement of the disappearance of NADH (Bergmeyer and Bernt 1955). The aspartate/cysteinesulfinate/cysteate reaction mixture contained 0.12 mmol NADH/L (50 mmol/L Hepes buffer, pH 7.4), 100 units of lactate dehydrogenase, 100 units of malate dehydrogenase, 200 µmol/L -ketoglutarate and substrate (L-aspartate, L-cysteinesulfinate or L-cysteate) at concentrations about 10 times their respective K_m values (Weinstein and Griffin 1988), 0.1 mL of tissue supernatant preparation in a final volume of 3 mL in the cuvette. The reaction was initiated by addition of -ketoglutarate. Correction was made for blanks run concurrently without any substrate (amino acid). Absorbance of NADH at 340 nm was measured spectrophotometrically at 25°C during the linear part of the reaction. All reagents were purchased from Sigma Chemical Company, (St. Louis, MO) except cysteinesulfinic acid and cysteic acid which were purchased from United States Biochemical Corp., (Cleveland OH).

Liver and intestinal mucosa preparation.  Samples of liver and upper small intestine were resected from four adult cats and four adult Sprague Dawley rats and placed on ice. Mucosal tissue from the small intestine was obtained by scraping off the mucosa using a glass slide. Tissue samples weighing 0.5-1.0 g were homogenized with either 4 volumes (small intestine) or 9 volumes (liver) of double deionized water to assure release of mitochondrial transaminases. The homogenates were centrifuged at 30,000 × g for 30 min prior to removal of supernatant for assaying activities of aminotransferase with the various substrates.

Statistics.  The effect of dietary treatment on mean body weight and food intake was analyzed by ANOVA followed by Duncan's multiple range test. Comparisons of significance of differences due to diet were evaluated using ANOVA for repeated measures (Cody et al. 1987). For these analyses SAS statistical software (SAS Institute, Cary, NC) was used. The fecal and urine taurine values were analyzed as two-way ANOVA using Sigmastat© version 2 which also tested for normality and equal variances. In cases where these tests failed, values were transformed before analysis. Differences were accepted as significant at P < 0.05 (Steel and Torrie 1990).

	RESULTS

Abstract Introduction Methods Results Discussion References

Animal weight and food consumption.  No significant differences (P > 0.05) were found among dietary treatments in weekly body weight gain or daily food intake (results not shown). Food consumption by cats averaged 79.3 ± 16.4 g/d (mean ± SEM). Body weight gain over the 14-wk experiment was 0.35 ± 0.09 g/d.

Initial taurine status.  As expected the diet containing the resin caused a rapid decrease in the concentration of taurine in plasma, whole blood, and muscle (Figs. 1-3). By wk 3 the concentrations of taurine in plasma, whole blood, and muscle decreased to approximately 0.20, 0.49 and 0.37 of each initial value, respectively. After the resin was withdrawn from the diet, the plasma, whole blood, and muscle taurine significantly rebounded towards pre-treatment levels (P < 0.05) in groups fed diets containing TAU or TAU + CA, but not in the cats given the CSA diets.

View larger version (27K): [in this window] [in a new window]   Fig 2. Concentrations of taurine in the whole blood of cats given diets containing either taurine (TAU), cysteinesulfinic acid (CSA) or cysteic acid (CA). Values are means ± SEM, n = 5.

View larger version (20K): [in this window] [in a new window]   Fig 3. Concentrations of taurine in the muscle of cats given diets containing either taurine (TAU), cysteinesulfinic acid (CSA) or cysteic acid (CA). Values are means ± SEM, n = 5.

Taurine status after feeding cysteinesulfinic acid TAU and cysteic acid.  The effect of the various dietary treatments (wk 4-15) on the concentration of taurine in plasma, whole blood, and muscle are shown in Figures 1-3. Cats consuming diets containing CSA had a progressive decline in the concentration of taurine in plasma, whole blood, and muscle during the 12-wk period of dietary treatment. Mean whole blood concentration of taurine in cats fed the CSA diets was less than that of cats given the diet without taurine during wk 5-11 (Fig. 2). Similarly, plasma and muscle concentrations of taurine in cats given diets containing CSA or 0 TAU were not significantly different (P > 0.05) (Figs. 1 and 3). By 6 wk, cats receiving 0.4 g TAU/kg diet had concentrations of taurine in plasma significantly (P < 0.05) lower than cats given the diets containing either 0.8 g TAU/kg diet or 0.4 g TAU + 1.0 g CA/kg diet, but 40% higher than cats given diets without taurine. There was no significant difference (P > 0.05) in the concentration of taurine in plasma, whole blood, or muscle of cats given the 0.8 g TAU/kg diet and those given the diet containing 0.4 g TAU + 1.0 g CA/kg diet. The addition of 1.0 g CA/kg to the diet containing one-half the taurine required for maintenance of adequate taurine levels in the tissue (0.4 g TAU/kg diet), increased the concentrations of taurine in the plasma, whole blood, and muscle (Figs. 1-3). Cats fed the diet with 0.4 g TAU + 1.0 g CA had numerically higher concentrations of taurine in both plasma and muscle than cats given 0.8 g TAU/kg diet, but the differences were not significant (P > 0.05), (Figs. 1 and 3).

Excretion of cysteinesulfinic acid and taurine in the urine and feces.  Urine and feces of cats had very low and not significantly different (P > 0.05) quantities (pmol/d) of cysteinesulfinic acid. Urinary and fecal outputs (pmol/day) by treatment groups were 0 TAU: 77 ± 48 and 132 ± 63; 0.98 CSA: 103 ± 48 and 94 ± 35; 1.96 CSA: 94 ± 39 and 101 ± 44; 0.4 TAU: 95 ± 28 and 157 ± 72; 0.8 TAU: 113 ± 43 and 135 ± 61; 0.4 TAU + 1.0 CA: 102 ± 33 and 131 ± 58. Detection limit for cysteinesulfinic acid was 20 ± 10 pmol/d, and recovery of cysteinesulfinic acid was greater than 0.92 in spiked samples. If background levels of cysteinesulfinic acid are subtracted, then only 0.03 and 0.01% of dietary intake of cysteinesulfinic acid was recovered in urine of cats consuming diets containing 0.98 and 1.96 g CSA/kg diet, respectively, while cysteinesulfinic acid recovered in the feces was between 0.01 and 0.02% of cysteinesulfinic acid intake.

The effect of the dietary treatments on urinary taurine excretion are presented in Table 2. Most of the taurine in urine was in the bound state. No significant differences (P > 0.05) in urinary output of free and total taurine were found among dietary treatments after 1 wk of ingestion of the purified diet containing no resin. The urine of cats, after consuming the diet containing resin for 2 wk, had 0.20 of the free and 0.16 of the total taurine excreted initially, when cats were fed the same diet lacking the resin, but containing 1.5 g TAU/kg diet. Despite the depressed urinary levels of free and total taurine from cats consuming the resin diet for 2 wk, this treatment did not alter the percentage of bound taurine. Free and total urinary taurine significantly increased (P < 0.02) when either TAU or a combination of TAU and CA was added to the diet. Cats fed diets containing 0.4 g TAU + 1.0 CA/kg excreted significantly greater (P < 0.02) amounts of both free and total taurine in urine than cats given any other dietary treatment. This observation is in agreement with the plasma and blood taurine values and indicates that cats given the 0.4 g TAU + 1.0 g CA diet synthesized more taurine than the cats given the 0.8 g TAU diet. Likewise, cats ingesting diets containing 0.8 g TAU/kg diet excreted ~0.16 and 0.37 times more free and total taurine, respectively, than cats ingesting the 0.4 g TAU/kg diet. After 12 wk of receiving dietary treatments, the amount of bound taurine was significantly (P < 0.02) lower in the urine of cats ingesting diets containing taurine.

The fecal taurine excretion of cats fed diets containing various levels of cysteinesulfinic acid, taurine, and cysteic acid is presented in Table 3. The ingestion of the resin diet containing no taurine resulted in a decreased output of taurine in the feces, similar to that observed in urine. The significantly (P < 0.05) lower quantity of free and total taurine in feces approximated half the taurine in feces when cats received the purified diet containing 1.5 g TAU/kg diet for 1 wk. Cats ingesting diets devoid of taurine, maintained this low fecal taurine excretion at wk 6 and 12. On the other hand, cats fed all diets containing taurine had significantly (P < 0.05) greater amounts of fecal taurine. Cats given the diets with 0.8 g TAU/kg diet and 0.4 g TAU plus 1.0 g CA/kg diet had significantly higher (P < 0.05) amounts of free and total taurine in the feces than cats ingesting any of the other diets. Cats consuming the 0.4 g TAU/kg diet did not have a significantly (P > 0.05) greater free and total taurine excretion compared to cats ingesting diets without taurine. No apparent difference among dietary treatments was observed on the percentage of taurine bound in the feces.

Transaminase activities in liver and small intestinal mucosa.  The transaminase activities in liver and intestinal mucosa for aspartate, cysteinesulfinate and cysteate are presented in Table 4. When the activities of the liver and intestinal enzymes for cats and rats were analyzed in a two-way ANOVA, the activities for the three substrates were significantly higher (P < 0.001) in the liver than in the intestine, and also higher in cats than in rats. The transaminase activity for cysteinesulfinate was about three to five times that for cysteate, which has implications in regard to the relative contributions of cysteic acid and cysteinesulfinic acid to taurine biosynthesis.

	DISCUSSION

Abstract Introduction Methods Results Discussion References

We were unable to demonstrate in vivo that cats could synthesize taurine from dietary cysteinesulfinic acid. Cats given cysteinesulfinate in the diet had similar concentrations of taurine in plasma, whole blood, and muscle after 12 wk as cats given a taurine-free diet (Figs. 1-3). The route of administration appears to play an important role in the conversion of cysteinesulfinate to taurine in rats. Stipanuk and Rotter (1984) reported that i.p. administration resulted in conversion of cysteinesulfinic acid to taurine, but no taurine was detected from oral administration. These authors also found no label in taurine from cysteinesulfinate administered by intragastric intubation, but when L-[¹⁴C] cysteinesulfinate was given by i.p. injection to the rats, approximately 0.70 was converted to taurine. In a similar study with mice, Griffith (1983) demonstrated an in vivo partitioning whereby ~85% administered L-[¹⁴C] cysteinesulfinate was decarboxylated to hypotaurine and ~15% was transaminated. Griffith stated, "of the hypotaurine formed, approximately 90% is oxidized to taurine," although it is presumed that eventually all hypotaurine is converted to taurine. Results from another in vivo study in rats is consistent with the findings that i.p. injected L-[¹⁴C] cysteinesulfinic acid is converted to labeled taurine (Yamaguchi et al. 1978). These authors observed that 0.70 of L-[¹⁴C] cysteine was metabolized via the cysteinesulfinic acid pathway to taurine.

In our study, the lack of conversion of dietary cysteinesulfinic acid to taurine suggests that cat intestine, and or liver, efficiently transaminate cysteinesulfinate. Total liver and small intestinal mucosal aminotransferase activities (i.e., activities of supernatants after homogenizing tissues in water to release all mitochondrial transaminases) were higher in cats than in rats for all three substrates (aspartate, cysteate and cysteinesulfinate). Presumed near-maximal activities were measured with each substrate by using concentrations of each substrate 10 times each respective K_m (Weinstein and Griffith 1988). The greatest activity in cats, as in rats, was with cysteinesulfinate followed by aspartate then cysteate which supported the lowest rate of reaction.

Since we found no increase of cysteinesulfinate in urine or feces, it appears that all of the cysteinesulfinate in the diet was metabolized by the cat's tissues or by the microbes in the gut. Cysteinesulfinate, a normal intermediate of cysteine catabolism does not accumulate in tissues, but is rapidly metabolized by two competing pathways. The partitioning between these two pathways is presumably controlled by the relative activities of the transaminase and decarboxylase in the liver and possibly tissues other than the small intestine (Singer and Kearney 1956). While the product of the decarboxylation pathway leads to hypotaurine and taurine, the transamination pathway is essentially irreversible, since the product -sulfinyl pyruvate spontaneously decomposes to pyruvate and sulfite. The latter is oxidized to sulfate by sulfite oxidase.

In an earlier experiment we demonstrated that dietary cysteic acid, can efficiently provide all the taurine needed for cats, and in this present study at least half the dietary taurine for cats and adequately maintain whole body taurine status (Fig. 2). Several factors contribute to cysteic acid being a more efficient precursor of taurine for cats than cysteinesulfinate: the apparent K_m of cysteinesulfinate for transamination is lower in rats, and presumably in cats, than cysteate (Weinstein and Griffith 1988). More important, cysteate undergoes reversible transamination to -sulfopyruvate which is relatively stable, (Weinstein and Griffith 1988) is not excreted in the urine and so may be reaminated and decarboxylated to supply taurine. In contrast, the product of transamination of cysteinesulfinate is irreversibly degraded to pyruvate and sulfite.

The taurine requirement of cats fed purified diets can be met with either 800 mg taurine or 2.0 g cysteic acid/kg diet dry matter (Edgar et al. 1994). In our present study, the diet containing 0.4 g TAU + 1.0 g CA/kg maintained concentrations of taurine in the plasma and whole blood that were equivalent to cats given the diet containing 0.8 g TAU/kg and resulted in higher concentrations of taurine in muscle after 3 wk (Figs. 1-3). In addition, this diet produced the greatest quantity of free and total taurine in the urine, indicating these cats had a completely adequate taurine status and were excreting excess taurine due to down regulation of renal taurine resorption. These results indicate that cysteic acid could be used to supplement diets to provide all or part of the taurine in the diet for cats.

In summary, cats do not utilize dietary cysteinesulfinic acid as a precursor for taurine, whereas cysteic acid may replace part, or all of the dietary taurine needs of cats. The metabolic basis for the lack of utilization of dietary cysteinesulfinate appears to be efficient transamination to -sulfinyl pyruvate, which undergoes irreversible breakdown to pyruvate and sulfite rather than decarboxylation leading to taurine. In contrast, cysteic acid is reversibly transaminated to -sulfopyruvate, which is relatively stable and allows for the efficient decarboxylation of cysteic acid to taurine.

	ACKNOWLEDGMENTS

The feline vitamin mixture was a gift from Hoffman-La Roche, Nutley, NJ and is gratefully acknowledged. We thank The Upjohn Company of Kalamazoo, MI for the gift of Colestipol® HCl.

	FOOTNOTES

Manuscript received 10 February 1997. Initial reviews completed 21 March 1997. Revision accepted 9 October 1997.

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Abstract Introduction Methods Results Discussion References

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