Lysine nutrition in swine and the related monogastric animals: muscle protein biosynthesis and beyond

Abstract

Improving feed efficiency of pigs with dietary application of amino acids (AAs) is becoming increasingly important because this practice can not only secure the plasma AA supply for muscle growth but also protect the environment from nitrogen discharge with feces and urine. Lysine, the first limiting AA in typical swine diets, is a substrate for generating body proteins, peptides, and non-peptide molecules, while excess lysine is catabolized as an energy source. From a regulatory standpoint, lysine is at the top level in controlling AA metabolism, and lysine can also affect the metabolism of other nutrients. The effect of lysine on hormone production and activities is reflected by the change of plasma concentrations of insulin and insulin-like growth factor 1. Lysine residues in peptides are important sites for protein post-translational modification involved in epigenetic regulation of gene expression. An inborn error of a cationic AA transporter in humans can lead to a lysinuric protein intolerance condition. Dietary deficiency of lysine will impair animal immunity and elevate animal susceptibility to infectious diseases. Because lysine deficiency has negative impact on animal health and growth performance and it appears that dietary lysine is non-toxic even at a high dose of supplementation, nutritional emphasis should be put on lysine supplementation to avoid its deficiency rather than toxicity. Improvement of muscle growth of monogastric animals such as pigs via dietary lysine supply may be due to a greater increase in protein synthesis rather than a decrease in protein degradation. Nevertheless, the underlying metabolic and molecular mechanisms regarding lysine effect on muscle protein accretion merits further clarification. Future research undertaken to fully elucidate the metabolic and regulatory mechanisms of lysine nutrition could provide a sound scientific foundation necessary for developing novel nutritional strategies to enhance the muscle growth and development of meat animals.

Keywords: Amino acid; Epigenetic regulation; Gene expression; Lysine; Metabolism; Monogastric animal; Muscle; Protein modification; Protein synthesis; Swine.

Figure 1

A general formula of a peptide molecule. This representation shows at least one peptide bond. R₁, R₂, and R_n represent side chains of n amino acid residues. With a loss of one molecule of water from two amino acids, one peptide bond (−CONH−) is formed. When n = 0 (i.e., the third amino acid residue does not exist), the peptide will be a dipeptide; when n = 1, the peptide will be a tripeptide; and so on.

Figure 2

A general formula of lysine. Lysine is a cationic or basic amino acid with an α-amino group, a long side chain, and an ε-amino group. Presented in this formula is an ionized form of lysine.

Figure 3

Lysine catabolism in monogastric animals. Lysine is catabolized via the saccharopine pathway and the pipecolate pathway. The enzymes involved include: (1) Lysine α-ketoglutarate reductase; (2) saccharopine dehydrogenase; (3) lysine oxidase; (4) spontaneous; (5) peperideine-2-carboxylic acid reductase; (6) pipecolate oxidase; (7) spontaneous; (8) enzymes including aminoadipate semialdehyde dehydrogenase, aminoadipate aminotransferase, α-ketoacid dehydrogenase, glutaryl-CoA dehydrogenase, glutaconyl-CoA decarboxylase, enol-CoA hydratase, and β-hydroxyacyl-CoA dehydrogenase for seven steps; (9) thiolase; (10) enzymes of the TCA cycle. Adapted from Wu (2013a).