There have been many changes in the interest of amino acids in livestock. In the past, the function of amino acids as a component that constitutes proteins was examined. However, as biochemical technologies have developed in many fields such as cell culture, the physiological functions of amino acids have been realised and have to be considered in designing feed formulation to maximise the growth of the animal.
Use of methionine in poultry diets
Since methionine is limited in plant protein sources and is required at high level for feather growth and protein synthesis, methionine is always classified as the first limiting amino acid in poultry. Methionine has many physiological functions, e.g. as an important methyl donor to provide the methyl group (CH4) necessary for metabolism in the body. Methionine is also known to reduce oxidative stress in the body by increasing antioxidative compounds such as glutathione.
Methionine additives used in poultry and other feeds are L-methionine, DL-methionine or MHA-FA which is the hydroxyl analogue of methionine. All plants and animals can only utilise L-isomer methionine and only L-type methionine is present in proteins. D-isomer of methionine and MHA can also be used, but it must be converted into the L-form through enzyme reaction in the body (Figure 1). However, it can be questioned whether the cleaved amino group can be returned to the methionine chemical skeleton at 100% efficiency. Additionally, the 2 enzymatic reactions during the isomer conversion process require energy. Therefore, when DL methionine or MHA is used in place of L-methionine, the relative bioavailability (RBA) is naturally lowered due to unnecessary energy consumption for the enzymatic conversion.