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Article

High Dietary Protein and Taurine Increase Cysteine Desulfhydration in Kittens^{1 ,2}

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

⁴To whom correspondence and reprint requests should be addressed.

	ABSTRACT

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The objective of this study was to determine the effect of dietary protein and taurine on cysteine desulfhydration in various kitten tissues. Cysteine desulfhydration was assessed in liver, kidney, skeletal muscle, heart, spleen, brain and jejunum of kittens fed one of the following diets for 5 wk: 20% protein, 0% taurine diet (LP0T); 20% protein, 0.15% taurine diet (LPNT); 60% protein, 0% taurine diet (HP0T); and 60% protein, 0.15% taurine diet (HPNT). Cats fed LP0T and HP0T had been fed a taurine-free diet for 10 wk before the 5-wk experiment. The activity of cysteine desulfhydration was determined by measuring the production of H₂³⁵S from ³⁵S-cysteine in the presence and absence of -ketoglutarate (KG) in the incubation medium. Liver and kidney had the highest total activities among the tissues tested (P < 0.01). Total hepatic desulfhydration activities [µmol H₂S/(min · kg body wt)] in cats fed LP0T, LPNT, HP0T and HPNT were (mean ± SEM) 117 ± 6, 135 ± 10, 137 ± 10 and 190 ± 9, respectively. Dietary taurine had a significant effect on activity when expressed per gram liver (P < 0.01), per gram protein (P < 0.05) and per kilogram body weight (P < 0.001). Dietary protein had a significant effect (P < 0.001) only when activity was expressed relative to body weight because of the significant effect of protein on relative liver weight. The direct pathway via cysteine desulfhydrase appears to be the major route of cysteine desulfhydration in kitten liver because the values obtained in the absence of KG were 81–88% of those obtained in the presence of KG.

KEY WORDS: • cysteine desulfhydration • cysteine desulfhydrase • kittens • dietary protein • dietary taurine

	INTRODUCTION

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In most mammalian tissues, cysteine is metabolized to cysteine sulfinic acid (CSA),⁵ which is either decarboxylated to hypotaurine and then oxidized to taurine (Bagley and Stipanuk 1995 , Daniels and Stipanuk 1982 , Griffith 1983 , Stipanuk et al. 1994 ) or transaminated to its -keto acid, ß-sulfinylpyruvate which is readily converted to pyruvate and sulfite (Griffith 1983 ) (CSA pathway). Alternatively, cysteine undergoes desulfhydration, a process that is achieved largely by two pathways. One is the direct desulfhydration, which is catalyzed by cysteine desulfhydrase (EC 4.4.1.1) and releases the amino group of cysteine as ammonium ion (Goswami et al. 1959 , Simpson and Freedland 1976 , Yamaguchi et al. 1973 ). The second pathway involves the transamination of cysteine via cysteine aminotransferase (EC 2.6.1.3) in conjunction with the reaction catalyzed by ß-mercaptopyruvate sulfurtransferase (EC 2.8.1.2) (Stipanuk and Beck 1982 , Stipanuk and King 1982 , Taniguchi et al. 1984 , Ubuka et al. 1977 , Wlodek et al. 1993 ). In both pathways, the end products are pyruvate and hydrogen sulfide (H₂S).

Most of the cysteine catabolic enzymes have been reported to be regulated by dietary changes in protein, methionine and cysteine (Bagley and Stipanuk 1995 , Daniels and Stipanuk 1982 , Kohashi et al. 1978 , Simpson and Freedland 1976 , Stipanuk 1979 ), as well as by developmental stage (Kuo et al. 1983 , Loriette and Chatagner 1978 , Pasantes-Morales et al. 1976 ). The activity of rat liver cysteine dioxygenase (EC 1.13.11.20) was positively correlated with the dietary intakes of protein (Bagley and Stipanuk 1994 , Kohashi et al. 1978 ) and sulfur amino acids (Bagley and Stipanuk 1995 , Daniels and Stipanuk 1982 , Kohashi et al. 1978 , Stipanuk 1979 , Yamaguchi et al. 1985 ). High protein or high cysteine diets also significantly increased the activity of cysteine desulfhydrase in chick (Goswami et al. 1959 ) and rat liver (Simpson and Freedland 1976 ). On the other hand, hepatic cysteine sulfinic acid decarboxylase (EC 4.1.1.29) activity was reported to be significantly reduced by high protein or methionine-supplemented diets in rats (Bagley and Stipanuk 1994 , Jerkins et al. 1989 , Jerkins and Steele 1992 ), and increased by taurine depletion in kittens, presumably as a part of an adaptive response (Rentschler et al. 1986 ).

Investigations of the relative role of each pathway in cysteine catabolism in mammalian tissues have been sparse, and the results have led to inconsistent conclusions. In apparent conflict with the general consensus that the CSA pathway plays a major role in cysteine metabolism in rats, several observations suggest that a substantial portion of cysteine catabolism in rats occurs via a CSA-independent pathway, which leads to the formation of pyruvate and H₂S (De La Rosa et al. 1987 , Simpson and Freedland 1975 , Stipanuk et al. 1992 ). In cats, which have low activities of cysteine dioxygenase and cysteine sulfinic acid decarboxylase, compared with other species (Rentschler et al. 1986 ), the desulfhydration pathway appears to play an even more important role than the CSA pathway in cysteine catabolism (De La Rosa et al. 1987 ).

Many amino acid and nitrogen catabolic enzymes in the cat are not up- or down-regulated (Rogers et al. 1977 ) as occurs in omnivores and herbivores. Total sulfur amino acids are commonly the most limiting amino acid in cat foods. Both cats and humans have a limited ability to synthesize taurine from cysteine; thus cats may be a good model for humans for the study of sulfur amino acid metabolism. Nevertheless, only limited data exist on the activities of enzymes involved in various pathways of sulfur amino acid metabolism in various tissues of cats. Therefore, the objectives of the present study were as follows: 1) to evaluate the tissue distribution of cysteine desulfhydration activity in kittens; 2) to assess the relative significance of each desulfhydration pathway in cysteine catabolism; and 3) to determine the effect of dietary protein and taurine on cysteine desulfhydration in various kitten tissues.

	MATERIALS AND METHODS

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Animals and diets.

Female specific pathogen–free kittens (n = 24, 1773 ± 60 g, 16–20 wk old) from the Feline Nutrition and Pet Care Center of our Department were randomly selected from a larger group fed a purified diet containing 435 g soybean protein and 1.5 g taurine/kg diet. Half of the kittens were prefed a 400 g protein + 1.5 g taurine/kg diet for 1 wk; the other half were prefed a taurine-free diet containing 400 g protein/kg diet for 10 wk for the depletion of body taurine. Kittens fed the 400 g protein, 1.5 g taurine/kg diet were switched to either 200 g protein, 1.5 g taurine/kg diet (low protein, normal taurine, LPNT) or 600 g protein, 1.5 g taurine/kg diet (high protein, normal taurine, HPNT), and those fed 400 g protein, 0 g taurine/kg diet were switched to either 200 g protein, 0 g taurine/kg diet (low protein, 0 taurine, LP0T) or 600 g protein, 0 g taurine/kg diet (high protein, 0 taurine, HP0T) at the end of the prefeeding period (2 x 2 factorial design). Each of the four groups had six kittens. The compositions of the experimental diets are shown in Table 1 . All diets were fed in the form of pellets.

Blood and tissue collection.

Blood samples were collected from the jugular vein of unanesthetized cats into heparinized 3-mL syringes each week at 0900–1100 h without prior food deprivation. Liver, kidney, skeletal muscle, heart, spleen, brain and small intestine (jejunum) were removed from the kitten under pentobarbital anesthesia and immediately placed on ice. The contents of the intestine were washed out with ice-cold saline before the intestine was placed on ice. A portion of fresh tissue was homogenized in 0.05 mol/L potassium phosphate buffer, pH 6.8, using a Polytron homogenizer (model PT 10/35, Brinkman Instrument, Westbury, NY) set at #6 speed to form a 200 g/L homogenate. Homogenates were centrifuged at 20,000 x g for 30 min at 4°C and the supernatants were kept at -80°C until enzyme assays were conducted.

Enzyme assays.

Activity of cysteine desulfhydration was assayed radiochemically by measuring the rate of H₂³⁵S production from L-[³⁵S]-cysteine. The contribution of each pathway of cysteine desulfhydration was evaluated from the production of H₂S in the presence and absence of -ketoglutarate (KG). Because KG is involved only in the transamination reaction, activities obtained in the absence of KG represent direct desulfhydration, which is catalyzed by desulfhydrase and releases the amino group of cysteine as ammonium ion, whereas those obtained in the presence of KG represent total desulfhydration. Desulfhydration via the transamination of cysteine coupled with ß-mercaptopyruvate sulfurtransferase was obtained by difference.

The assay mixture contained 60 mmol/L L-cysteine, 37 kBq L-[³⁵S]-cysteine (DuPont NEN, Boston, MA), 2 mmol/L pyridoxal 5'-phosphate (PLP), 5 mmol/L KG, 10 mmol/L dithiothreitol (Sigma Chemical, St. Louis, MO), 100 mmol/L potassium phosphate buffer (pH 7.3) and 0.05–0.15 mL of the supernatant of 20% tissue homogenate in a final volume of 2 mL. The pH of each reagent was adjusted to 7.3. The assay mixture was placed in test tubes on ice; the tubes were flushed with N₂ gas for 20 s and immediately sealed with rubber stoppers. Two 0.7 x 2.5 cm² pieces of Whatman no. 1 filter paper soaked with methylbenzethonium hydroxide solution (Sigma Chemical) were hung from a hook on the rubber stopper to trap H₂³⁵S produced. The reaction was initiated by placing the tube in a 37°C shaking water bath and terminated by the addition of 1 mL of 0.92 mol/L trichloroacetic acid through a needle penetrating the rubber stopper. After the termination of the reaction, test tubes remained at 37°C for 1 h to allow all of the H₂S to be trapped in the methylbenzethonium hydroxide. The radioactivity trapped in filter paper was determined by liquid scintillation spectrometry (model 2000 A, Packard Instrument,Downers Grove,IL) using 15 mL of scintillation cocktail (Liquiscint, National Diagnostics, Somerville, NJ). Reagent blanks were tested in the same manner except that tissue supernatant was replaced by the buffer solution.

Analytical methods.

A portion of the heparinized blood was centrifuged at 3000 x g for 10 min for separation of plasma. Both whole blood and plasma were analyzed for taurine using an amino acid analyzer (model 121-MB, Beckman Instrument, Palo Alto, CA). DNA content was determined in the tissue homogenates spectrophotometrically by the method by Burton (1956) and protein by the method of Lowry et al. (1951) .

Statistical analyses.

The values in the tables and figures represent the mean ± SEM of 6 kittens. Overall comparisons among the four groups were done by using two-way ANOVA to test effects of dietary levels of protein, taurine and their interactions (2 x 2 factorial analysis). Changes in plasma and whole-blood taurine concentrations within groups over time were tested by repeated-measures two-way ANOVA. When variances associated with each experimental mean were unequal, data were log-transformed before analysis. All statistical comparisons were made using SAS computer program (SAS/STAT Version 6, SAS Institute, Cary, NC).

	RESULTS

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At the end of the 5-wk period, kittens fed HP0T and HPNT gained 380–460 g, whereas those fed LP0T and LPNT lost 30–80 g of body weight compared with their initial body weights (Fig. 1 ). Liver weights were significantly higher in the cats fed the high protein diets than in those fed low protein diets, i.e., 67% greater for those fed the normal taurine diets and 112% greater for those fed the taurine-free diets. The ratios of liver weight to body weight also were greater in kittens fed HPNT (30%) and HP0T (71%) compared with the values for kittens fed LPNT and LP0T (P < 0.001; Table 2 ). DNA concentration was not significantly affected by the dietary concentration of protein or taurine (Table 2) .

View larger version (21K): [in this window] [in a new window]   Figure 1. Effect of a high protein diet and taurine depletion of kittens on cumulative body weight gain for 5 wk. Each bar represents mean ± SEM, n = 6. Diet abbreviations: low protein, 0 taurine diet (LP0T); high protein, 0 taurine diet (HP0T); low protein, normal taurine diet (LPNT); and high protein, normal taurine diet (HPNT). Weight gains at wk 5 were significantly affected by dietary protein concentration (P < 0.001), but not by dietary taurine concentration (two-way ANOVA). Protein x Taurine interaction was not significant.

View larger version (35K): [in this window] [in a new window]   Figure 2. Effect of a high protein diet and taurine depletion of kittens on tissue distribution of cysteine desulfhydration. Total activity of cysteine desulfhydration was measured in kittens fed low protein, 0 taurine diet (LP0T) and high protein, normal taurine diet (HPNT). Each bar represents mean ± SEM, n = 6. Abbreviations used: B, brain; H, heart; I, small intestine; K, kidney; L, liver; M, skeletal muscle; and S, spleen. Significant differences between means (Student’s t test) for LPOT and HPOT within a tissue were found only for liver (P < 0.05), muscle (P < 0.01), and heart (P < 0.05). Different superscripts within a group indicate differences (P < 0.001) among tissues.

A significant positive taurine effect on hepatic cysteine desulfhydration that was independent of the mode of expression of the activity, i.e., per gram liver (P < 0.01), per gram liver protein (P < 0.05) or per kilogram body weight (P < 0.001) was observed. The activity of hepatic cysteine desulfhydration, expressed per kilogram body weight, was increased by the dietary intake of protein (P < 0.001), a result of the increased liver size per unit body weight in the kittens in the high protein group. There were no differences when expressed per gram of liver protein (P > 0.05).

	DISCUSSION

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The low protein diets used in this study did not support the growth of kittens (Fig. 1) . The protein requirement for maximal growth for kittens is 24% when the diet exceeds all of the essential amino acid requirements (NRC 1986 ). The low protein diet was limiting in total sulfur amino acids, providing only about two thirds of the minimal requirement for maximal growth. Nevertheless, it is surprising that the kittens did not grow at a slow rate (Rogers et al. 1990 , Schaeffer et al. 1982 ). Requirements for essential amino acids are higher when kittens are fed a diet below the nitrogen (protein) requirement (Rogers et al. 1990 ); thus the low essential/dispensable amino acid ratio in this diet may have contributed to the lack of growth.

Ingestion of the taurine-free diets for 15 wk reduced the plasma taurine concentration of kittens to ~1 µmol/L regardless of the protein content in the diet. It has been repeatedly observed in our laboratory that high protein diets, especially high soybean protein diets, significantly reduce plasma taurine concentrations (Kim et al. 1995 ).

Results from the study on tissue distribution of cysteine desulfhydration showed that overall, cysteine desulfhydration was most active in the liver followed by kidney (Fig. 2) . It is noteworthy that the rest of the tissues also had relatively high activities of cysteine desulfhydration (34–48% of the activity found in the liver). Considering the large proportion of skeletal muscle in the whole body, it would appear that skeletal muscle may play an important role in the cysteine desulfhydration. These results from our study agree with previous findings on tissue distribution of transaminative and desulfhydrase activities in rats. By determining the thiocyanate formed from L-cysteine and cyanide, Ishimoto (1979) reported that the overall activity of the transaminative pathway of cysteine catabolism in rats was most active in liver, followed by the kidney and heart. Stipanuk et al. (1982) found the highest activity of overall transaminative pathway in the heart and liver in cats and rats. Cysteine desulfhydrase activity was also measured in various rat tissues, and the liver and kidney had the highest activity of cysteine desulfhydrase among the tissues tested (Yamaguchi et al. 1973 ). Our values of H₂S production, obtained in the presence of 60 mmol/L cysteine with PLP, KG and dithiothreitol, were ~2–4 times those reported by Stipanuk and King (1982) in various cat tissues. Perhaps direct comparisons should not be made because the activity is from more than one enzyme and the conditions were somewhat different.

Activity of cystathionase toward disulfides of cysteine was reported in a mutant strain of Neurospora (Flavin and Segal 1964 ) and in mammalian livers (Cavallini et al. 1960 , Yao et al. 1979 ). However, because a reducing agent (dithiothreitol) was added to the incubation medium, the conversion of cysteine to cystine, the substrate of cystathionase for desulfhydration, would be minimal.

We found that 81–88% of total cysteine desulfhydration activity came from the reaction catalyzed by cysteine desulfhydrase, thus <20% of the activity came from the transamination of cysteine via cysteine aminotransferase coupled with ß-mercaptopyruvate sulfurtransferase. Few studies have examined the quantitative importance of each desulfhydration pathway in cysteine catabolism in mammalian tissues. Kuo et al. (1983) and Stipanuk and Beck (1982) concluded that cysteine desulfhydration obtained in the presence of cysteine, PLP, KG and dithiothreitol was due primarily to cysteine aminotransferase plus ß-mercaptopyruvate sulfurtransferase activities in rat and cat tissues because omission of KG from their assay mixture resulted in a 92–96% and 100% decreases in the activity of cysteine desulfhydration in the liver of rats and cats, respectively. However, in a more recent study using freshly isolated rat hepatocytes, Drake et al. (1987) suggested that cysteine was not a good substrate for transamination, and little transamination of cysteine with KG occurred in rat hepatocytes. Our results in kittens support the idea that the pathway via cysteine desulfhydrase accounts for the major route of hepatic cysteine desulfhydration in kittens.

Fifteen weeks of taurine depletion significantly decreased the specific activity of hepatic cysteine desulfhydration (Table 5 ). This would appear to be advantageous because when the kittens are depleted of taurine, more cysteine is metabolized through the CSA-dependent pathway that is the major route of taurine biosynthesis. However, depending on the composition of the diet fed, only ~10% of the taurine required by the kitten can be synthesized in vivo (Hickman et al. 1992 ); thus the physiologic importance of this adaptation is questionable. Goswami et al. (1959) observed that the activity of cysteine desulfhydrase per gram liver in chicks fed low protein (3% casein + 1.3% gelatin) and protein-free diets was 75 and 58%, respectively, of that found in chicks fed the control diet (18% casein + 10% gelatin). Similarly, Simpson and Freedland (1976) reported that rats fed 90% protein diet had a twofold greater activity of cysteine desulfhydrase per gram liver compared with those fed a nonpurified control diet. In this study, protein concentration in the diet did not significantly affect the activity of cysteine desulfhydration when expressed per gram liver. However, cysteine desulfhydration activity per kilogram body weight was significantly increased by the high protein diet (because of the greater liver size) as well as by taurine supplementation. An increased ratio of liver weight to body weight, after the consumption of high protein diets for 5 wk (Table 2) , appears to contribute to the enhanced capacity of hepatic cysteine desulfhydration in kittens.

	FOOTNOTES

 
¹ Presented in part at the Waltham International Symposium on Nutrition of Small Companion Animals, September 1990, University of California, Davis, CA [Park, T., Jerkins, A. A., Steele, R. D., Rogers, Q. R. & Morris, J. G. (1991) Effect of dietary protein and taurine on enzyme activities involved in cysteine metabolism in cat tissues. J. Nutr. 121: S181–S182].

² Supported by the George and Phyllis Miller Feline Health Fund, Center for Companion Animal Health, School of Veterinary Medicine, University of California, Davis, CA.

³ Current address: Department of Food and Nutrition, Yonsei University 134 Shinchon-dong, Sudaemun-ku, Seoul, Korea.

⁵ Abbreviations used: KG, -ketoglutarate; CSA, cysteine sulfinic acid; HPNT, high protein, normal taurine diet; HP0T, high protein, 0 taurine diet; LPNT, low protein, normal taurine diet; LP0T, low protein, 0 taurine diet; PLP, pyridoxal phosphate.

Manuscript received March 15, 1999. Initial review completed May 25, 1999. Revision accepted August 24, 1999.

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