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Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5;
* The Rowett Research Institute, Aberdeen, Scotland, UK;
Department of Surgery, Faculty of Health Sciences, McMaster University, Hamilton, ON, Canada L8S 4L8; and
** Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1
3To whom correspondence should be addressed. E-mail: cdelange{at}uoguelph.ca.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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25 kg BW, the pigs’ PDmax was determined using the N-balance method. Thereafter, 4 pigs were allocated to each of 4 diets, first-limiting in lysine, providing lysine intakes corresponding to 60, 70, 80, and 90% of estimated requirements for PDmax. The pigs were surgically fitted with catheters in the jugular and femoral veins. Lysine catabolism was determined at 2 BW (40–45 kg, low; 70–75 kg, high) either directly (oxidation) using a primed, constant infusion of L-[1-14C]-lysine or indirectly (disappearance) using the N-balance method. There was no effect of BW on the rate (g/d) or fraction of TID lysine intake catabolized. Lysine catabolism decreased with increasing growth potential. Lysine disappearance and lysine oxidation (% of TID lysine intake) were independent of lysine intake, except for the lowest lysine intake level, where they were lower. When lysine catabolism was independent of intake, lysine oxidation based on plasma free lysine specific radioactivity (SRA) was lower (9.9% of TID intake) than lysine disappearance (17.4% of TID intake) or lysine oxidation based on liver free lysine SRA (13.4% of TID intake).
KEY WORDS: • protein deposition • nitrogen balance • lysine intake • lysine catabolism • lysine oxidation
Amino acid catabolism contributes to the inefficiency of utilizing dietary amino acids for protein deposition (PD)4 in growing animals. Some inevitable amino acid catabolism occurs even when amino acid intake limits PD. This appears to reflect the presence of amino acid–catabolizing enzymes in animal cells (1).
Estimates of inevitable lysine catabolism in growing pigs range from 3% (2) up to 40% of the pig’s lysine intake (1). Most estimates of lysine catabolism are between 15 and 30% of available lysine intake (3–5). The wide range of these estimates may reflect biological variation or methodological difficulty in measuring amino acid catabolism. Potential contributors to biological variation are body weight (BW) (6), varying amino acid intake levels (7–10), and the pig’s performance potential. Different experimental methods, such as direct determination of amino acid oxidation by isotope tracers or indirect determinations based on serial slaughter or N-balances, may result in different estimates of amino acid catabolism (11).
In particular, the rate of amino acid catabolism at varying levels of amino acid intake below the animal’s requirements for maximum body protein deposition is unclear. Young et al. (9) suggested that a constant quantity of amino acids is inevitably catabolized daily. Alternatively, inevitable catabolism may be a constant fraction (5,7,12) or an increasing fraction of amino acid intake (10), implying that the daily amount oxidized increases with amino acid intake. These different responses have substantial implications for estimates of amino acid requirements.
The objective of the current study was to determine the influence of lysine intake, BW, and growth performance potential on lysine catabolism in growing pigs. Catabolism was estimated both directly, using an isotope tracer, and indirectly by N balance. Lysine catabolism was measured in pigs receiving 4 levels of lysine intake at 40 and 80 kg BW.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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15 kg BW and housed individually in a temperature-controlled room (19–22°C). In the first phase of the experiment, the pigs consumed ad libitum a diet designed to allow expression of maximum PD (PDmax, Table 1). At 25 kg BW, a 5-d N-balance study was conducted to determine the individual pig’s PDmax (13). This was followed by the second phase of the experiment, during which lysine catabolism was determined when the pigs were fed restrictively. The pigs were randomly allocated to 4 diets that were formulated to provide sufficient lysine to support 60, 70, 80, or 90% of their mean PDmax (L60, L70, L80, and L90, respectively). At 35 kg BW, the pigs were surgically fitted with catheters in the jugular and femoral veins (14). After at least 1 wk of recovery and at a mean BW of 40 kg (low BW), L-[1-14C]lysine was infused for 8 h into the femoral catheter for the direct determination of lysine catabolism (oxidation). The catheters were then removed and a N-balance study was conducted for 5 d at a mean BW of 45 kg for the indirect determination of lysine catabolism (disappearance). The pig’s N-balance was determined again at 70 kg (high BW). The pigs were again surgically fitted with jugular and femoral catheters for determining lysine oxidation, as at the low BW. At the end of the infusion, the pigs were killed by an intravenous injection of pentobarbital and liver samples were taken. The pigs were weighed weekly throughout the experiment.
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Isotope infusions. The isotope infusion studies were performed in an open-circuit respiration system as described previously (19). The isotope infusion was primed with 10 kBq NaH14CO3 and L-[1-14C]lysine at 1.5 times the hourly infusion rate of L-[1-14C]lysine. This dose was injected manually via the femoral catheter within a 1-min period. Immediately thereafter, the constant infusion was started using variable speed syringe pumps (Harvard Apparatus). The infusion rates were 398.7 ± 7.5 and 674.0 ± 12.6 kBq/h L-[1-14C]lysine for the low and high BW, respectively. Both NaH14CO3 and L-[1-14C]lysine (declared purity > 99%) were purchased from American Radiolabeled Chemicals.
The CO2 in the exhaust air was trapped quantitatively by an absorber (ethanolamine:2-methoxyethanol, 1:2). The CO2 absorber was weighed and sampled every 30 min throughout the 8-h infusion period. Blood samples (4 mL) were taken from the jugular catheter at 30-min intervals and 15 min after CO2 was sampled. The blood samples were transferred into heparinized tubes. The plasma was separated by centrifugation for 10 min at 3000 x g, and stored at –20°C until analysis. At the end of the second infusion at the high BW, samples of the right central liver lobe were taken immediately after death, frozen in liquid N2, and stored at –20°C until further analysis.
Analytical procedures. The dry matter content of feed was determined by oven drying for 2 h at 135°C (20). The N content was determined by the Kjeldahl method in dried samples of nutrient and energy mixes and in fresh samples of urine (20). Lipid contents (petroleum ether extraction) and fiber contents were determined according to the AOAC (20). Heats of combustion of feed and feces were determined in an adiabatic bomb calorimeter (20). Dietary amino acid concentrations were determined by Degussa A.G., using ion-exchange chromatography with postcolumn derivatization with ninhydrin (21,22), except for tryptophan, which was determined by HPLC with fluorescence detection (extinction 280 nm, emission 356 nm), after alkaline hydrolysis with barium hydroxide octahydrate for 20 h at 110°C (22).
Plasma and liver free lysine concentrations were determined by HPLC (Waters) after precolumn derivatization with phenylisothiocyanate (PITC) (23) using norleucine as an internal standard. To determine the recovery of radioactivity, U-[14C]-leucine was used as a radioactive internal standard and the HPLC eluates containing the leucine and lysine fractions were collected for subsequent scintillation counting. In plasma (1 mL), protein was precipitated by the addition of 0.5% trifluoroacetic acid in methanol and centrifuged. The supernatant fraction was freeze-dried and made alkaline with 1 mL of methanol:water:triethylamine (20:60:20). The samples were derivatized for 35 min at room temperature using 300 µL of a 5% PITC solution, freeze-dried and reconstituted in phosphate buffer (5 mmol disodium phosphate with 5% (v) acetonitrile, pH 7.4). Liver samples (1 g) were frozen in liquid N2, and homogenized in 5 mL ice-cold perchloric acid (2%), and centrifuged for 10 min at 3000 x g. The supernatant was applied to 1.5 mL of cation-exchange resin (AG 50W-X8, Biorad Laboratories). Contaminants were removed by washing the columns with 10 mL of distilled water. Amino acids were eluted with 6 mL of 5 mol/L ammonium hydroxide, and dried. The remaining preparation for HPLC was the same as for the plasma samples. To determine radioactivity in plasma and liver free lysine, and in the CO2 absorber, the scintillation counter (Beckman LS 6000, Beckman) was set to count for 30 min or a 2
error of 1% (19). The counting efficiency was 
90% for CO2 samples and U-[14C]-leucine fractions and 70–80% for other samples. The actual counting errors were 1–2% for CO2 samples and U-[14C]-leucine fractions and 4–7% for other samples.
Calculation of results. The N-balance results were calculated assuming a fecal N-digestibility of 96.6 (SE = 0.08, n = 317) derived from previous experiments with similar pigs and diets (11,13). Lysine disappearance was calculated as the true ileal digestible lysine (TID) intake minus lysine retained in body protein (PD x 0.0708, SE 0.0018, n = 50) minus physical lysine losses via gut and integument. Lysine disappearance, therefore, represents an indirect measure of lysine catabolism. The gut endogenous lysine losses were calculated as 142 + 0.8 x dry matter intake –0.00013 x (dry matter intake)2 (24). Lysine losses in integument were estimated as 4.04 mg/kg0.75/d (25).
For determination of lysine oxidation, the SRA (Bq/µmol) of plasma free lysine was in the first instance assumed to be similar to that at the site of lysine oxidation. The oxidation rate (µmol/0.5 h) was calculated as expired 14CO2 divided by plasma free lysine SRA. The 14CO2 excretion was corrected for the recovery of CO2 in the pigs (84.7%) (19) and for the recovery of radioactivity during sample preparation. Daily oxidation rates were calculated according to Möhn et al. (19) using the second complete feeding-to-feeding period when all pigs had reached a steady state in oxidation. Lysine oxidation rates were expressed as daily rates (g/d) and as fractions of TID lysine intake (%). Lysine oxidation rates based on liver free lysine SRA were obtained by dividing the daily rates of oxidation for each pig by the mean, constant ratio of liver free lysine SRA to plasma free lysine SRA at the end of the infusion.
Statistical analysis. Least square means ± SE were derived using general linear models (26) with BW, TID lysine intake, and PDmax as sources of variation. Least square means were compared pair-wise using Tukey’s test. Interactions between the main sources of variation were tested and included in the final model if P < 0.1. Differences between methods were assessed comparing overall means using Student’s t test. Relations between lysine oxidation and lysine disappearance, and lysine catabolism and PDmax were assessed using Pearson’s correlation coefficients. Differences were considered significant at P < 0.05.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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At the low BW, pigs consumed 23.9 ± 0.2 MJ/d of ME at a BW of 43.5 ± 0.5 kg. At the high BW, pigs consumed 28.6 ± 0.1 MJ/d of ME at a BW of 71.2 ± 0.7 kg. The small differences in ME intake between groups did not affect outcome measurements. Lysine intakes were as intended (Table 2). PD and calculated lysine disappearance did not differ between the 2 BW (data not shown). Therefore, all results were combined for low and high BW. For the 2 BW, PD increased linearly (P < 0.05) with lysine intake, whereas it was greater (P < 0.05) in L80 and L90 than in L60 and L70 (Table 2). Lysine disappearance, both as a daily rate and as a fraction of intake, increased linearly and quadratically with lysine intake, whereas values were lowest in L60 (P < 0.05) and did not differ among L70, L80, and L90. Lysine intake, however, did not affect lysine disappearance after exclusion of group L60. Lysine disappearance for L70, L80, and L90 was 17.4 ± 1.3% of TID lysine intake or 2.10 ± 0.17 g/d.
Observed PDmax at 25 kg BW influenced (P < 0.05) PD and lysine disappearance when pigs were fed the lysine-limiting diets. Across lysine intake levels, PD increased linearly by 0.27 ± 0.12 g/g PDmax, whereas lysine disappearance, as a fraction of TID intake and daily rate decreased by 0.17 ± 0.007%/g PDmax and 0.020 ± 0.008 g/g PDmax. The daily rate of lysine oxidation decreased with increasing PDmax (P < 0.001) by 0.025 ± 0.006 g/g increase in PDmax, whereas the fraction of TID lysine intake oxidized decreased (P < 0.001) by 0.25 ± 0.05%/g PDmax. When data from L60 were excluded from the regression analyses, lysine disappearance tended (P = 0.08) to decrease by 0.15 ± 0.08 g/g and 0.018 g/g PDmax (P = 0.08) as a fraction of TID and daily rate, respectively. Lysine oxidation, excluding L60, decreased (P = 0.03) by 0.21 ± 0.06 g/g and 0.025 ± 0.07 g/g PDmax as a fraction of TID and daily rate, respectively.
Although the plasma free lysine SRA decreased with TID lysine intake at the low body weight, it was independent of TID lysine intake at the high body weight (Table 3). Liver free lysine SRA was not affected by TID lysine intake. Therefore, the ratio of liver free to plasma free lysine SRA was not affected by TID lysine intake, and a constant ratio of 0.74 was assumed to calculate lysine oxidation rates based on liver free lysine SRA.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Common to both methods was the absence of an influence of BW on lysine catabolism. This agrees with results reported by Bikker et al. (6), Möhn et al. (11), and Susenbeth et al. (27). In addition, the experimental design allowed us to directly relate inevitable lysine catabolism to the pigs’ protein deposition potential. Pigs that exhibited a higher PDmax at 25 kg BW subsequently had a lower rate of lysine catabolism, and vice versa, whether group L60 was included or excluded from the data analysis. Pigs in this group with the lowest lysine intake had exhibited the greatest PDmax, which may have confounded the results. This indicates that lysine utilization efficiency increases with pig performance potential and seems to support the observed positive correlation between the efficiency of protein utilization and performance traits in growing pigs (28). This also agrees with the observation that increased PD in somatotropin-treated growing pigs is accompanied by a reduction in amino acid oxidation (29). These data also suggest that direct genetic selection for reduced inevitable lysine catabolism may be possible and desirable.
When calculated from N-balance data, lysine disappearance is a residual value, dependent on assumptions about the lysine content in PD and lysine losses with skin, hair, and gut endogenous protein. Lysine disappearance may not reflect actual lysine catabolism. Moreover, N-balance methods result in an overestimation of PD, and thus in an underestimation of lysine disappearance compared with a serial slaughter assay (11). Mean lysine disappearance at moderately restricted TID lysine intakes (groups L70, L80, L90; 2.1 g/d or 17.4% of TID lysine intake) was greater than lysine oxidation based on plasma free lysine SRA (1.2 g/d or 9.9% of TID lysine intake), which is consistent with previous observations in our laboratory (19). Assuming that the ratio of liver to plasma free lysine SRA was similar at low and high BW, lysine oxidation (1.6 g/d or 13.4% of TID lysine intake) calculated on the basis of free liver lysine SRA still tended to be lower than lysine disappearance. This discrepancy may be caused by assumptions about lysine usage for other body functions when calculating lysine disappearance. Moreover, SRA in liver free lysine may not accurately represent SRA in the actual precursor pool for lysine oxidation or recovery of 14CO2 may not be estimated accurately based on an infusion of NaH14CO3 (19).
Lysine catabolism, whether calculated from N-balances or from tracer kinetics, was lowest for group L60. This may be caused by a conservation of lysine at low intakes as suggested by Kim et al. (30), which agrees with the improved efficiency of lysine utilization at low intakes reported by Gahl et al. (8). However, the lysine flux based on plasma free lysine SRA for group L60 at 40 kg body weight (5.1 g/d) was obviously unrealistic because it was less than the lysine intake (8.9 g/d), and much lower than for the other groups (12.2–26.0 g/d). This implies that the actual lysine oxidation rate in L60 was greater than measured. Greater oxidation rates based on the lysine SRA of a precursor pool more appropriate than plasma would be accompanied by a greater flux, thus abolishing the apparent discrepancy between lysine flux and intake. For both these reasons, the results of group L60 are omitted from the following discussion. Excluding group L60 from the analyses, our results for both lysine disappearance and oxidation suggest that a constant fraction of lysine intake is catabolized at moderate levels of restriction (L70, L80, and L90). This agrees with the recent findings of Heger et al. (31) in growing pigs. In contrast, Heger and Frydrych (10) proposed that the fraction of lysine catabolized in growing rats increases with lysine intake, whereas Brookes et al. (7) suggested that in growing rats, lysine catabolism was a constant fraction of lysine intake. The data set generated by Fuller and Garthwaite (32) can be interpreted to support both the concepts of a constant or an increasing fraction of amino acid intake being catabolized with increasing level of amino acid intake. Interpretations of the relations between lysine catabolism and lysine intake are clearly influenced by mathematical models that are applied to the data and by the apparent lysine-sparing effect at severely restricted lysine intakes (group L60). At less severely restricted intakes (L70, L80, and L90), however, lysine catabolism appears to account for a constant fraction of intake. The observation that lysine catabolism represents a constant fraction of intake at moderate restrictions of lysine intake is consistent with constant fractional inevitable threonine catabolism at moderately limiting threonine intake levels (33), and with the linear PD response to intakes of essential amino acids, when amino acid intake limits PD (3,31,34).
Möhn et al. (19) argued that the marginal efficiency of utilizing available lysine intake for PD is directly and inversely related to lysine catabolism because an insignificant amount of lysine is used for the synthesis of nonprotein compounds. Reported values for the marginal efficiency of lysine retention in PD derived from serial slaughter data are between 71 and 86% of TID lysine intake (3,5,11). The observed lysine oxidation of 13% at the 3 highest lysine intake levels in the current experiment, and based on liver free lysine SRA, represents a constant marginal efficiency of using TID lysine intake for PD of 87% when lysine intake approaches requirements for PDmax. This efficiency is slightly higher than estimates obtained in serial slaughter studies, suggesting that the isotope tracer method as used in the current study likely underestimates actual inevitable lysine catabolism.
In conclusion, on the bases of N-balance observations and isotopic tracer studies, inevitable lysine catabolism was not influenced by BW in growing pigs fed lysine supplied from casein between 40 and 80 kg BW. Both experimental approaches showed that the rate of inevitable lysine catabolism decreases with increasing pig growth potential (measured as maximum PD at 25 kg BW). The reduced rate of lysine catabolism at the lowest lysine intake level (40% below requirements for maximum PD) can be interpreted as a lysine-sparing effect when lysine intake is restricted severely. At moderate lysine intake restrictions (10–30% below requirements for maximum PD), lysine catabolism appeared to be independent of lysine intake and amounted to 16.7% of the true ileal digestible lysine intake based on N-balance observations. Based on isotope tracers, and assuming that liver free lysine SRA represents the level of labeling at the site of lysine catabolism, 13.2% of the lysine intake was catabolized in growing pigs. This indicates a constant marginal utilization of true ileal digestible lysine intake for PD when lysine intake approaches requirements for maximum PD; this marginal efficiency was estimated to be 
87%.
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FOOTNOTES |
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2 Supported by Archer-Daniels Midland, Degussa AG, Eurolysine SpA., Rhone Poulenc Animal Nutrition, Ontario Ministry of Agriculture, Food and Rural Affairs, and the Natural Science and Engineering Research Council. S.M. was supported by a scholarship from the Deutsche Forschungsgemeinschaft.
4 Abbreviations used: BW, body weight; ME, metabolizable energy; PD, whole-body protein deposition; PDmax, maximum whole-body protein deposition; PITC, phenylisothiocyanate; SRA, specific radioactivity; TID, true ileal digestible.
Manuscript received 9 February 2004. Initial review completed 8 March 2004. Revision accepted 23 June 2004.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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1. Moughan, P. J. (1991) Towards an improved utilization of amino acids by the growing pig. Haresign, W. Cole, D.J.A. eds. Recent Advances in Animal Nutrition 1991:45-64 Butterworth-Heinemann Oxford, UK. .
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30. Kim, K. I., Elliot, J. I. & Bayley, H. S. (1983) Oxidation of an indicator amino acid by young pigs receiving diets with varying levels of lysine or threonine, and an assessment of amino acid requirements. Br. J. Nutr. 50:391-399.[Medline]
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ustala, M. & imeèek, K. (2003) Efficiency of amino acid utilization in the growing pig at suboptimal levels of intake: branched-chain amino acids, histidine and phenylalanine + tyrosine. J. Anim. Physiol. Anim. Nutr. 87:52-66.[Medline]
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34. Heger, J., Van Phung, T. & Kíová, L. (2003) Efficiency of amino acid utilization in the growing pig at suboptimal levels of intake: lysine, threonine, sulphur amino acids and tryptophan. J. Anim. Physiol. Anim. Nutr. 86:153-165.
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