Methionine in Poultry Diets & Its Role in Antioxidant Protection – Part II

Most of these stressors can induce acute stress responses, reducing production performance, health and impacting animal welfare. As well as hampering the quality and quantity of meat products in poultry flocks.

Some stressors may increase susceptibility to diseases, leading to reduced daily gains and poorer feed conversion rates. A common source of stress is the excessive generation of free radicals, causing oxidative stress at the molecular level.

Methionine residues can be used as endogenous antioxidants to help alleviate damage caused by excess reactive oxygen species.

Similarly, dietary methionine can upregulate mRNA expression of heat shock proteins in the intestine, protecting against intestinal mucosal damage in broiler chickens under heat stress. Thus, dietary methionine can help mitigate adverse reactions caused by high-temperature stress.

Methionine is a precursor of succinyl-CoA, homocysteine, cysteine, creatine, and carnitine. It is also a precursor for cellular methylation and cysteine synthesis, reducing the dietary requirement for cysteine.

It can combat oxidative stress as a precursor to antioxidants or by incorporating residues into proteins.

Reactive oxygen species (ROS) are produced by various physiological and non-physiological events, including the Fenton reaction, cellular respiration, mitochondrial dysfunctions, pathologies, phagocytes, neutrophils, and stress.

Methionine plays an essential role in the immune system through its metabolites. It directly influences immune function as its catabolism can increase the production of glutathione, taurine, and other metabolites. Methionine is also readily utilized by hepatocytes for direct glutathione synthesis.

Figure 1. Antioxidant/free radical mechanism of action.

The most common and studied antioxidants in different tissues of poultry concerning methionine supplementation include:

• Total antioxidant capacity (TAC)
• Superoxide dismutase (SOD)
• Catalase (CAT)
• Glutathione peroxidase (GPX)
• Glutathione (GSH)

Among the enzymatic substances, superoxide dismutase (SOD) is responsible for catalyzing the dismutation of the superoxide anion (O2-) into less reactive species like hydrogen peroxide (H2O2). The formation of the superoxide anion involves oxygen with two unpaired electrons binding to another electron-sharing molecule:

O2 + e- → O2-

Dismutation is the process through which this oxide radical, upon receiving hydrogen ions, results in hydrogen peroxide. This compound is then reduced by catalase (CAT) and glutathione peroxidase (GPx) enzymes:

2O2- + 2H+ → H2O2

Thus, these enzymes act as the first line of antioxidant defense in the body, helping control free radical production. The end result of their action is harmless molecules like oxygen and water.

2H2O2 + CAT → 2H2O + O2

Methionine protects against oxidative stress in two ways: first, by indirectly producing antioxidants (precursors of cysteine and glutathione) and second, through Met residues in proteins (Figure 2). The latter pathway occurs during repair, where various ROS products attack Met residues in proteins to form Met sulfoxide, generating cells that eliminate reactive species.

Methionine sulfoxide reductases (MSRs) are enzymes that reduce ROS in all organisms, from bacteria to mammals. They accomplish this by oxidizing eight Met residues per protein molecule, maintaining the integrity of each enzyme.

Figure 2. Overview of stressor effects, antioxidant system function, and Met influences on the antioxidant system/status in poultry. Adapted from Lugata, 2022.

Thioredoxin (TRx) prepares the enzyme for another catalytic cycle by reducing the oxidized MSR. NADPH is also an eliminator of reactive nitrogen and oxygen species.

The antioxidant potential via this pathway is influenced by heat stress and methionine supplementation, where TRxR1 and MsrA genes were highly expressed in broilers under heat stress supplemented with DL-HMTBA.