Methionine and lysine are the building blocks of casein synthesis, but they are unfortunately also known as limiting amino acids (AA) for milk protein metabolism in ruminants, while casein derivatives mediate anti-oxidative actions
[42]. Studies on the effects of dietary inclusion of rumen protected methionine and lysine on oxidative status
[43], immune response reproduction, and milk performance
[44] of dairy cattle have been well documented, but limiting AA have been tested as immune function regulators in dairy cows during early lactation
[45]. Moreover, little information exists on optimum inclusion levels and synergetic effects of rumen protected amino-acids on their oxidative status Mavrommatis et al. (2021)
[46] recently found that a decrease in plasma BHBA led to increased glutathione peroxidase (GSH-Px) activity in lactating ewes that were provided a combination of rumen protected methionine and lysine. Therefore, increasing antioxidants activity has a beneficial effect on animal health and can decrease the incidence rate of metabolic disorder diseases like ketosis. Furthermore, AA contribute to cellular oxidative balance
[47] through participating in taurine and GSH synthesis
[48], ensuring cellular detoxification and hydrogen peroxide neutralization via glutathione S-transferase and GSH-Px actions, respectively. Methionine plays a key role in de novo short- and medium-chain FA synthesis as a source of methyl for the transmethylation reactions in lipids biosynthesis
[49]. It also enhanced de novo glutathione and carnitine synthesis in the liver, and thus increased antioxidant and β-oxidation capacity
[50]. Sun et al.
[51] reported that supplementation of methionine increase the “very-low-density lipoproteins” (VLDL) resulting in enhanced vitamin E circulation. He also demonstrated that dietary supplementation of rumen protected amino-acids can suppress side effects of lipid peroxidation by-products such as MDA. Moreover, higher FRAP values in blood plasma and milk of ewes fed combinations of rumen-protected Met + choline + betaine compared with the control were observed
[52]. Besides the inclusion of methionine and lysine and its benefit in enhancing antioxidants status,
N-carbamylglutamate (NCG, a metabolically stable analogue of
N-acetylglutamate synthase (NAG) that produces arginine endogenously) can also play a significant role in improving immune function and oxidative status in suckling lambs. Zhang et al. (2018)
[53] conducted a study to investigate effects of dietary supplementation with
l-arginine (Arg) and
N-carbamylglutamate (NCG) on intrauterine growth-retarded (IUGR) suckling lambs. They showed that the concentrations of protein carbonyl (PCs) and MDA were lower and the glutathione (GSH) concentration and ratio of GSH/GSSG greater in the jejunum, duodenum and ileum of IUGR + 1% Arg or 0.1% NCG lambs, compared to IUGR group. Zhou et al. (2016)
[54] isolated polymorphonuclear leukocytes (PMNL) and showed the lower abundance of genes linked to inflammation (IL1B, TLR2, NF-κB, and STAT3) and oxidative stress (CBS, GPx1, glutathione synthase [GSS], and SOD2) as well as an increase in plasma taurine with methionine provision, and proposed improved redox and inflammatory status of those cells. A recent trial conducted by Lopreiato et al. (2019)
[55] studied the consequences of incubation bovine PMNL with Met and/or choline and observed that methionine supplementation coupled with sufficient choline enhanced gene expression of TLR2 and
l-selectin, which are pathogen recognition mechanisms. In the same trial, cells incubated without choline had high mRNA abundances encoding IL1B, IL6, IL10, and myeloperoxidase (MPO), glutathione reductase (GSR), GSS, cystathionine gamma-lyase (CTH), and cysteine sulfinic acid decarboxylase (CSAD), suggesting higher inflammation and oxidative stress.