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Poult Sci 2006. 85:1932-1938
© 2006 Poultry Science Association
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METABOLISM AND NUTRITION

Conversion of the Methionine Hydroxy Analogue DL-2-Hydroxy-(4-Methylthio) Butanoic Acid to Sulfur-Containing Amino Acids in the Chicken Small Intestine1

R. Martín-Venegas*, P. A. Geraert{dagger} and R. Ferrer*,2

* Departament de Fisiologia, Facultat de Farmàcia, Universitat de Barcelona, 08028 Spain; and {dagger} Adisseo France S.A.S., 92160-Antony, France

2 Corresponding author: rutferrer{at}ub.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
DL-Methionine or its corresponding hydroxy analogue, DL-2-hydroxy-(4-methylthio) butanoic acid (DLHMB), are commonly added to commercial animal diets to satisfy the TSAA requirement. The utilization of DLHMB as a supplementary source of Met begins with its conversion to L-Met via a 2-step mediated process. L-Methionine can then be transsulfurated to L-Cys, which, in turn, can be catabolized to taurine (TAU). In the present study, the capacity of the chicken small intestine to convert DLHMB to L-Met and to use this amino acid as a source for L-Cys and TAU production was evaluated. The appearance of Met in the serosal compartment of everted sacs incubated with DLHMB is higher in the presence of an H+ gradient (mucosal pH 5.5 vs. 7.4). Serosal Cys and TAU concentration was compared in everted sacs incubated at a mucosal pH of 5.5 with DLHMB or L-Met, and the results show significantly higher values after incubation with the hydroxy analogue. Regional comparisons indicate no significant differences in the appearance of serosal Met and Cys, although lower values were obtained for TAU in the duodenum than in the jejunum and ileum. The profile of non-S amino acids was also determined and revealed no significant differences between DLHMB- and L-Met-incubated sacs. In conclusion, Cys and TAU content in chicken enterocytes is higher when DLHMB is used as a Met source.

Key Words: DL-2-hydroxy-(4-methylthio) butanoic acid • methionine hydroxy analogue • cysteine • taurine • chicken small intestine


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Synthetic sources of dietary Met, such as DL-Met or its corresponding hydroxy analogue, DL-2-hydroxy-(4-methylthio) butanoic acid (DLHMB), are commonly added to commercial animal diets to satisfy the TSAA requirement for growth and maintenance. The bioefficacy of DLHMB compared with DL-Met has been the subject of numerous studies (Thomas et al., 1991; Esteve-Garcia and Austic, 1993; Huyghebaert, 1993; Rostagno and Barbosa, 1995; Maenz and Engele-Schaan, 1996; Esteve-Garcia and Llauradó, 1997; Lemme et al., 2002) and remains controversial today. Differences in intestinal absorption and metabolism of DL-Met and DLHMB would be expected to affect the nutritional utilization of these 2 Met sources. Commercial DLHMB contains a significant proportion of nonmonomeric forms (Koban and Koberstein, 1984; Lawson and Ivey, 1986), which are poorly absorbed by the chicken intestine (Saunderson, 1991) and have a lower biopotency than the monomeric form (Van Weerden et al., 1992). However, we have previously compared in vivo and in vitro DLHMB absorption in the chicken intestine with a product containing only the monomer, and we found no differences resulting from the high hydrolytic capacity of the intestinal mucosa (Martín-Venegas et al., 2006).

Upon absorption, DLHMB must be converted into L-Met for effective utilization. Although there are extensive studies on the absorption of DLHMB, relatively few studies have examined its metabolism in the chicken intestine. The conversion of this synthetic source to the biologically active amino acid involves 2 enzymatic steps: oxidation of the {alpha}-carbon, followed by transamination. The first reaction in the conversion of DLHMB to L-Met is a stereo-specific oxidation involving different enzymes: peroxisomal L-2-hydroxy acid oxidase and mitochondrial D-2-hydroxy acid dehydrogenase, which catalyzes the oxidation of L-HMB and D-HMB, respectively (Dibner and Knight, 1984), thereby yielding the corresponding {alpha}-keto acid, 2-keto-(4-methylthio) butanoic acid (KMB). The specific enzyme L-2-hydroxy acid oxidase has been found in chicken liver and kidney (Gordon and Sizer, 1965), whereas D-2-hydroxy acid dehydrogenase has been detected in numerous tissues, including liver, kidney, skeletal muscle, intestine, pancreas, spleen, and brain (Dibner and Knight, 1984). After the formation of the common intermediate KMB, the second step is its conversion to L-Met by transamination, which is ubiquitous and does not constitute the limiting step in the complete conversion process of DLHMB (Harter and Baker, 1977; Knight and Dibner, 1984; Rangel-Lugo and Austic, 1998).

L-Methionine is a nutritionally indispensable amino acid needed for many important metabolic functions: 1) protein synthesis; 2) transmethylation to form S-adenosylmethionine, a primary methyl donor that methylates compounds to form such products as creatine and phosphatidylcholine and that also participates in polyamine synthesis; and 3) transsulfuration to form L-Cys, which in turn is also a precursor amino acid for protein synthesis that can be incorporated into glutathione or catabolized to taurine (TAU). As a glutathione precursor, L-Cys plays a key role in intestinal epithelial antioxidant functions, and it may also regulate epithelial cell proliferation via modulation of redox status (Shoveller et al., 2005). Taurine is involved in many physiological functions, including osmoregulation, detoxification, and antioxidation (Huxtable, 1992; O’Flaherty et al., 1997; Lambert, 2004; Roig-Pérez et al., 2005).

For many decades, the liver has been identified as the primary organ involved in dietary amino acid metabolism, with the role of the intestine restricted to digestion and the absorption of dietary constituents. Nevertheless, recent evidence supports the view that the intestinal epithelium obtains a substantial fraction of its metabolic energy from the catabolism of dietary amino acids and, moreover, processes absorbed amino acids before their entry into portal circulation, thereby determining their systemic availability (Brosnan, 2003; Young, 2004; Shoveller et al., 2005). Thus, it can be hypothesized that the conversion of DLHMB to L-Met and further intestinal metabolism may be involved in the nutritional efficiency of this Met source. For this reason, the aim of the present research was first to evaluate serosal Met appearance after DLHMB incubation in everted sacs from the chicken small intestine; second, to compare the serosal appearance of Cys and TAU after DLHMB and L-Met incubation; and, finally, to compare free amino acid patterns in response to incubation with both Met sources.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Materials
All reagents were purchased from Sigma-Aldrich Corp. (St. Louis, MO). Zoletil (tiletamine-zolazepam) was obtained from Virbac France S.A.S. (Carros, France). The DLHMB was supplied by Adisseo France S.A.S. as Rhodimet AT88 (88% of active substance, Adisseo France S.A.S., Antony, France).

Birds and Diets
Male Ross chickens (Gallus gallus domesticus L.), obtained from Granja Crusvi (Montblanc, Catalonia, Spain), were raised at standardized temperatures (26 to 28°C), humidity (40 to 60%), and light (16L:8D) at a density of about 1 chicken/500 cm2. The birds were fed ad libitum from hatching to d 18 to 21 with balanced diets (Table 1Go) formulated and prepared by the Institut de Recerca i Tecnologia Agroalimentàries, Generalitat de Catalunya, Reus, Catalonia, Spain. These diets were supplemented with DL-Met or DLHMB (Rhodimet AT88) on an equimolar Met basis, taking into account that DL-Met source is 99% pure and DLHMB source is 88% pure. Experiments with DLHMB or L-Met as substrate were performed on 18- to 21-d-old birds fed DLHMB or DL-Met, respectively. The experimental protocol was approved by the Experimental Animal Ethical Research Committee of the Universitat de Barcelona, in accordance with the current regulations for the use and handling of experimental animals (Decret 214/97, Generalitat de Catalunya).


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  		Table 1. Composition of the experimental diets
 
Transport Experiments
Everted sacs were prepared following Wilson and Wiseman (1954) as previously described (Martín-Venegas et al., 2006). Briefly, the chickens were anaesthetized with 60 mg/kg of Zoletil and killed by decapitation without previous starvation. A portion of the duodenum (pancreatic loop), jejunum (6 cm proximal and distal to Meckel’s diverticulum), and ileum (the region connected with mesentery to the ceca) was removed and immediately flushed with ice-cold saline solution (4°C). The intestinal segments were turned inside out and cut into 4 portions of about 3 cm in length. Each portion was tied at the ends to form a sac and was filled with the serosal medium and incubated for 30 min at 37°C in 15 mL of the mucosal medium, which was continuously gassed with carbogen (95% O2 and 5% CO2). At the end of the incubation, the sacs were dried, weighed, and their contents carefully drained. The empty sacs were then weighed again, and the decrease in weight was taken as the volume of fluid remaining after incubation. The serosal content was then centrifuged (16,000 x g for 5 min at 4°C), and the supernatant was stored at –80°C until measurement. The mucosal medium was a Krebs-Henseleit bicarbonate buffer, which contained the following (in mmol/L): 118 NaCl, 4.74 KCl, 1.18 MgSO4·7H2O, 1.27 CaCl2, 1.18 KH2PO4, 25 NaHCO3, and 7 DLHMB or L-Met, gassed with carbogen until pH 7.4. For the experiments performed at pH 5.5, bicarbonate was replaced by 2-(4-morpholino) ethanesulfonic acid, and pH was adjusted with Tris. In all the experiments, the serosal medium was the bicarbonate buffer (pH 7.4) without substrate. The pH of all solutions was maintained during incubation. Results were normalized to the weight of the empty sac after incubation and expressed as nmol/100 mg of tissue.

Amino Acid Quantification
Quantitative analysis of amino acids was carried out by ion exchange chromatography following the methodology described by Moore et al. (1958). The analyzer (amino acid analyzer, model Alpha Plus, Amersham Pharmacia LKB Biotech-Biochrom, Cambridge, UK) was equipped with a cation exchange column (sulphonate polystyrene-divinylbenzene resin, 5-µm particle size, and 200 x 4 mm in length and i.d., respectively; Biochrom, Cambridge, UK). Chromatographic runs were made by using the Li citrate buffer gradient and temperature gradient recommended by the manufacturer for physiological fluids. The eluate was mixed with ninhydrin-hydrindantin reagent, and the reaction with amino acids was allowed to run in a coil at 135°C. In these conditions, amino acids form colored derivatives with absorption peaks at 570 and 440 nm for amino acids and imino acids, respectively. The different amino acids were identified by their retention time, via comparison with an amino acid standard solution of known composition run in the same batch and were quantified by comparison of its peak areas with those of the standard.

Data Analysis
The results are reported as means ± SEM. All data were compared by ANOVA using SPSS software (SPSS Inc., Chicago, IL). The P-value P < 0.05 was considered to denote significance.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Methionine serosal appearance was determined in DLHMB-incubated sacs from the duodenum, jejunum, and ileum at a mucosal pH of 5.5 and 7.4, after 30 min of incubation. The results (Figure 1Go) show a higher serosal appearance in the presence of an H+ gradient along the chicken small intestine, although no significant differences were detected in the jejunum. The regional profile for each pH revealed no significant differences, except at a pH of 7.4, in which the results show lower values in the duodenum than in the jejunum and no differences among these segments and the ileum. Taking into account the pH effect observed, further experiments were only performed in the presence of an H+ gradient.


Figure 1
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  		Figure 1. Methionine serosal appearance in everted sacs incubated for 30 min with 7 mmol/L of DL-2-hydroxy-4-(methylthio) butanoic acid in the mucosal compartment in the presence (pHm 5.5 to pHs 7.4) or absence (pHm 7.4 to pHs 7.4) of an H+ gradient. The results are expressed as the mean ± SEM of n = 8 sacs for the duodenum and n = 13 sacs for the jejunum and ileum. Asterisks (*) indicate mean differences between pH conditions (P < 0.05). The regional profile revealed no significant differences, except at a pH of 7.4, in which the results show lower values in the duodenum than in the jejunum and no differences between these segments and the ileum (P ≥ 0.05).

 
The results of Cys serosal appearance (Figure 2Go) show, in the 3 intestinal segments, significantly higher values after DLHMB incubation than after L-Met incubation. Regarding the regional profile, statistical analysis indicated no significant differences (P ≥ 0.05) between intestinal segments in either substrate.


Figure 2
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  		Figure 2. Cysteine serosal appearance in everted sacs incubated for 30 min with 7 mmol/L of DL-2-hydroxy-4-(methylthio) butanoic acid (DLHMB) or L-Met in the mucosal compartment at a mucosal pH of 5.5. The results are expressed as the mean ± SEM of n = 7 sacs for the duodenum and n = 9 sacs for the jejunum and ileum. Asterisks (*) indicate mean differences between substrates (P < 0.05). For both substrates, the regional profile reveals no significant differences.

 
Taurine serosal appearance in the duodenum, jejunum, and ileum is shown in Figure 3Go. As with the Cys results, following DLHMB incubation, the data reveal significantly higher values than those obtained with L-Met. For both substrates, the duodenum shows the lowest values compared with the more distal regions.


Figure 3
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  		Figure 3. Taurine serosal appearance in everted sacs incubated for 30 min with 7 mmol/L of 2-hydroxy-4-(methylthio) butanoic acid (DLHMB) or L-Met in the mucosal compartment at a mucosal pH of 5.5. The results are expressed as the mean ± SEM of n = 7 sacs for the duodenum and n = 9 sacs for the jejunum and ileum. Asterisks (*) indicate mean differences between substrates (P < 0.05). For both substrates, the regional profile reveals the lowest values for the duodenum compared with the more distal regions (P < 0.05).

 
The serosal appearance of non-S amino acids, which are shown in the order that they appear in the chromatogram, showed no statistical differences in all the intestinal segments following DLHMB or L-Met incubation (Table 2Go), suggesting that amino acid serosal appearance was not affected by the Met source used. Comparison of the results shown in Figures 1Go, 2Go, 3Go and Table 2Go indicates that TAU is the more concentrated free amino acid in the serosal compartment. In contrast, the serosal concentrations of Met and Cys were lower than most of the other amino acids.


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  		Table 2. Non-S amino acid serosal appearance in everted sacs after DL-2-hydroxy-4-(methylthio) butanoic acid (DLHMB) or L-Met incubation1
 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Methionine is a limiting essential amino acid in animal diets. To meet the nutritional requirement of this amino acid, dietary supplementation with synthetic sources of Met, such as DL-Met or DLHMB, is a common practice in animal feed production. To be efficiently utilized, both DL-Met and DLHMB must be absorbed from the intestinal lumen and then metabolized to L-Met. The present study examined the serosal appearance of S amino acids in everted sacs as an indicator of their systemic availability.

Conversion of DLHMB to L-Met is a complex process involving different enzymatic mechanisms. Although studies performed with DLHMB as a substrate for L-Met synthesis have shown that the liver accounts for the majority of total body conversion, the kidney and small intestine, among other tissues, also substantially participate in DLHMB metabolism (Gordon and Sizer, 1965; Langer, 1965; Dupuis et al., 1989, 1990; Dibner and Ivey, 1992; Song et al., 2001; Wang et al., 2001). Therefore, some of the dietary DLHMB will already be converted to L-Met and utilized by the intestine, thus maintaining a favorable gradient for DLHMB absorption by preventing its intracellular accumulation (Dibner and Knight, 1984). In chickens, KMB conversion to L-Met has been discounted as the limiting step in this process (Harter and Baker, 1977; Knight and Dibner, 1984; Rangel-Lugo and Austic, 1998), because a wide variety of amino acids could serve as substrates for transamination (Rangel-Lugo and Austic, 1998). These authors reported intermediate activity of the liver and intestinal mucosa, because the kidney remains the most active organ. The results of the present study support the existence of substantial DLHMB conversion to Met along the chicken small intestine. Moreover, Met appearance after DLHMB incubation is higher in the presence of an H+ gradient. We have previously demonstrated, using the same experimental conditions, higher serosal DLHMB appearance at a mucosal pH of 5.5 (Martín-Venegas et al., 2006), which suggests a direct relationship between substrate transport and conversion to Met. In addition, we have also observed the lowest DLHMB serosal appearance values in the duodenum when compared with the jejunum and ileum. However, similar Met serosal appearance after DLHMB incubation was found (Figure 1Go), suggesting that the duodenum has a high capacity to convert DLHMB to L-Met. In this sense, Brachet and Puigserver (1992) described an important activity of the enzyme responsible for the conversion of D-Met to L-Met in the chicken small intestine, with the highest activity occurring in the duodenal mucosa.

L-Methionine, L-Cys, and TAU are metabolically linked via the unidirectional transsulfuration pathway. The cellular content of L-Cys and TAU in various tissues is thought to be maintained by their absorption and biosynthesis, as well as by their efficient removal (Shimizu and Satsu, 2000; Stipanuk, 2004). L-Cys and TAU homeostasis in response to dietary changes appears to be primarily maintained by the liver (Ide et al., 2002; Stipanuk et al., 2002), although the contribution of nonhepatic tissues to further L-Met metabolism should also be considered (Stipanuk et al., 2002). Indeed, the complete transsulfuration pathway is present not only in the liver but also in the kidney, small intestine, and pancreas (Finkelstein, 1998). Our results indicate that Cys and TAU serosal appearance differ with the Met source assayed, which suggests the contribution of the dietary S amino acid content to the regulation of L-Cys and TAU systemic availability immediately upon absorption. Regarding the regional profile, the appearance of Cys shows no differences. In contrast, TAU content, like Met serosal appearance, is significantly the lowest in the duodenum, following the same previously reported profile as DLHMB transport in the chicken small intestine (Martín-Venegas et al., 2006). All these data suggest that the intestinal metabolism of dietary DLHMB is nutritionally relevant. In fact, the sum of Met, Cys, and TAU appearing at the serosal compartment in DLHMB-incubated sacs, taking into account DLHMB appearing at the serosal compartment in the same experimental conditions (Martín-Venegas et al., 2006), reveals a conversion of DLHMB to S-containing amino acids of 71, 59, and 63 % in the duodenum, jejunum, and ileum, respectively, confirming the high contribution of the duodenum.

Rat small intestinal cells are very active in the metabolism of dietary L-Cys, because high levels of this amino acid adversely affect the viability of the enterocytes (Coloso and Stipanuk, 1989). In contrast, TAU is the most abundant intracellular free amino acid (O’Flaherty et al., 1997), possessing many protective cellular functions (Huxtable, 1992; Lambert, 2004). Therefore, it can be hypothesized that, in the chicken intestine, L-Cys might be diverted to TAU synthesis. In fact, Bella and Stipanuk (1995) reported that the formation of TAU vs. sulfate as the end product of L-Cys hepatic catabolism represents a metabolic compensation, minimizing the acid load in rats fed excess S amino acids. In this way, although metabolic L-Cys deviation to TAU synthesis in the intestine is described to be lower than in the liver (Coloso and Stipanuk, 1989), incubation with an organic acid, such as DLHMB, produces higher TAU levels than those detected with L-Met. In this way, DLHMB has not only a potential as a Met source, but it also could be more easily involved in the detoxification process through the transsulfuration pathway. Therefore, the contribution of the small intestine to the total body transsulfuration capacity should be taken into consideration. Moreover, the intestinal epithelium of chickens encounters hyperosmotic luminal fluids during the digestion process (Mongin, 1976), and 1 of the functions of TAU is thought to be osmoregulation (Wright et al., 1986; Huxtable, 1992). Shimizu and Satsu (2000) reported an increase in TAU content when Caco-2 cells, an intestinal human cell line, were exposed to hypertonic stress. Therefore, an increased level of TAU after DLHMB incubation, compared with L-Met, would be favorable to homeostasis maintenance in postprandial periods.

The intestinal epithelium is a highly dynamic system continuously renewed by a process involving cell proliferation and differentiation, and it obtains a substantial fraction of its metabolic energy from the catabolism of dietary amino acids. Additionally, the intestinal epithelium is known to catabolize a significant portion of dietary amino acids, including Met, thereby modulating the amino acid availability to other tissues (Wu, 1998). Thus, it becomes important to dispose of not only S amino acids but also of all other amino acids. For these reasons, free amino acid patterns in response to incubation with both Met sources were also determined. The results showed no differences in the intestinal free amino acid patterns following L-Met or DLHMB incubation, which suggests that the intake of either of these substrates does not compromise the availability of non-S amino acids. Accordingly, Song et al. (2001) described that the infusion of either DL-Met or DLHMB had no effect on hepatic metabolism of amino acids other than that of Met.

In summary, our results not only confirm the capacity of the intestine to convert DLHMB to Met, but they also show a direct relation between the transport of this hydroxy analogue and its conversion. Moreover, Cys and TAU synthesis after incubation with DLHMB is higher when compared with L-Met incubation. Therefore, the data indicate that Cys and TAU formation by chicken enterocytes could be favored when DLHMB is used as a Met source, thereby suggesting that the hydroxy analogue might be preferentially diverted to the transsulfuration pathway. Nevertheless, the mechanism underlying these differences warrants further investigation.


   ACKNOWLEDGMENTS
 
We thank Enric Esteve from Institut de Recerca i Tecnologia Agroalimentàries for diet preparation and discussions on efficacy of dietary Met sources. The valuable help of the Serveis Cientificotècnics of the Universitat de Barcelona is also gratefully acknowledged.


   FOOTNOTES
 
1 The present study was supported by project 3891 from the Fundació Bosch i Gimpera and Adisseo France S.A.S. and by grant 2005-SGR-0632 from the Generalitat de Catalunya. R. Martín-Venegas holds a Recerca i Docència fellowship from the Universitat de Barcelona. Back

Received for publication March 7, 2006. Accepted for publication June 1, 2006.


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 ABSTRACT
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 RESULTS
 DISCUSSION
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