Energy values of synthetic amino acids in feed formulation: the case of methionine sources

Nutritionists agree that livestock diets require supplementation with some essential amino acids to increase the efficiency of protein utilization and to reduce nitrogen excretion. Methionine is an essential amino acid and is necessary for growth. Numerous trials have shown that providing adequate levels of methionine are required to maximize performance of poultry and pigs. Additionally, methionine plays biological roles beyond protein synthesis, through conversion into other sulphur-containing metabolites such as glutathione and taurine, two major cellular non-enzymatic antioxidants, and as a donor of methyl-groups. Therefore, underestimating the importance of methionine in monogastric nutrition has an important economic impact.

To meet the methionine requirements of livestock, diets are supplemented with a synthetic source of methionine. There are currently three sources of methionine available for animal nutrition: L methionine (L-Met), DL-methionine (DL-Met) and DL-OH-Methionine (DL-OH-Met). Animals can only use L Met for protein synthesis and the other sources are precursors and need to be converted by the animal to L Met. Methionine sources are valued in feed formulation not only for the supply of sulphur amino acids (methionine and cysteine), but also for the energy supply. Yet, the energy values of synthetic methionine sources need to be clarified. This paper aims to determine the energy values of synthetic methionine sources based on their respective metabolic pathways. The study also calculates the energy cost of the different conversion steps leading to L-Met from D-Met and DL-OH-Met.

Crystalline L-Met, like other amino synthetic amino acids, is supposed to be 100% digestible and the digestible energy (DE) value is therefore equal to the gross energy (GE) value. If dietary L-Met is used for body protein deposition, its metabolizable energy (ME) value equals the DE and GE value. However, if (part of) L-Met is deaminated, part of the energy of L-Met will be lost as urea or as uric acid. Only the energy retained in the carbon chain can then be used for other purposes. In the INRA tables (Sauvant et al. 2004), it is assumed that pigs retain 50% of the dietary protein in body protein and that the other 50% is excreted in the urine. The value of 50% retention is probably a conservative estimate for L-Met (as a free amino acid) because it is included to reduce the crude protein content of the diet and to increase the efficiency of nitrogen utilization. L-Met has a GE value of 3522 kJ/mol (23.6 kJ/g), while the energy values for urea (2 N-atoms) and uric acid (4 N-atoms) are respectively 635 and 1926 kJ/mol. The ME value of L-Met can thus be calculated as:

Mammals: ME = p · 3522 + (1-p) · (3522 – 635/2)

Birds: ME = p · 3522 + (1-p) · (3522 – 1926/4)

The calculation of the net energy (NE) value of L-Met is more complex. If L-Met is retained as methionine in body protein, one could think that all of the energy of methionine is retained as well. Although this is true, it requires energy to retain methionine in body protein, because the synthesis of a peptide bond requires at least 4 ATP. Moreover, due to protein turnover, peptide bonds are hydrolysed and resynthesized repeatedly. The hydrolysis of a peptide bond also requires at least 1 ATP. Table 1 shows the energy cost to retain 1 mole of methionine.

The turnover of body protein varies greatly among tissues (e.g., it ranges from 4% per day in muscle to more than 100% per day in intestinal tissues). It is difficult to provide a theoretical estimate of the energy cost of amino acid turnover. When L-Met is deposited in muscle early on in life, it is likely that it underwent 6 turnover cycles by the time the animal is slaughtered 150 days later (0.04 x 150 days). It is quite similar in poultry where the muscle turnover is faster (20% per day) but for a shorter duration (35 days) resulting in 0.20 × 35 = 7 cycles. On the other hand, an amino acid given a few days before slaughter may only require the ATP cost of peptide synthesis to be retained in muscle protein.

Empirical NE estimates can be used to calculate the NE-to-ME ratio of L-Met. For example, it is often assumed that the NE-to-ME ratio of dietary protein is approximately 60%. Based on the amino acid composition of the whole-body protein, this would correspond to an average of three turnover cycles (i.e., from weaning to slaughter and across tissues). This would mean that the NE-to-ME ratio would be 67% (Table 1). This value is greater than that of whole-body protein because L-Met is a relatively large amino acid.

Once deaminated, the carbon-chain of methionine can be used for different purposes, such as the synthesis of non-essential amino acids, lipid, or ATP. Experimental evidence from van Milgen et al. (2001) indicated that the NE-to-ME ratio was remarkably similar for a well-balanced protein source (casein, most of which is retained as body protein) compared to an unbalanced protein source (zeïn, most of which will be deaminated and used for other purposes). Although this result was coincidental, it allows us to hypothesize that the NE-to-ME ratio of protein and amino acids is not affected by the ultimate fate of the protein and amino acids. Energy values of L-Met assuming that 75% of L-Met is retained as body methionine are presented Table 2.

DL-Methionine contains 50% of D-Met and 50% of L-Met whereas OH-Met contains 50% D OH-Met and 50% L- OH-Met. The first step from methionine precursors to L-Met is the conversion to the common intermediate 2?keto-4-methylthio-butanoic acid (KMB). The conversion involves enzymes (oxidases and dehydrogenases), cofactors and release compounds, which differ depending on the precursor considered.

Crystalline amino acids are considered to be absorbed at 100% so that no faecal energy is lost and GE = DE. The ME value is linked to the urinary and gas energy losses. It is assumed that these energy losses are only linked to nitrogen excretion of the amino group (-NH2) from amino acids (Figure 1a). Accordingly, the ME-to-GE ratio is specific for the considered species and depends on the energy cost linked to urea or uric acid excretion in respectively mammals and birds.

The NE-to-ME ratio accounts for energy losses related to the metabolic costs of the conversion. Therefore, the calculation of the NE values considers the energy lost or spared in the conversion of the enantiomers of both precursors of L-Met (D-Met; D and L-OH-Met). This calculation is based on the ATP used or yielded from the direct or side reactions. To translate this into an energy value, we considered that one glucose molecule (2,820 kJ/mol) yields 31 ATP. Therefore, 91 kJ of glucose is required to synthesize 1 ATP. Accordingly, the calculation of energy losses is based on the equivalency of 91 kJ per ATP. This work was based on the metabolic model described by Van Milgen (2002). 

To simplify: