Methionine is the first limiting amino acid (AA) in poultry and one of the first limiting amino acids in other species. DL-methionine became commercially available and became a popular source of methionine after being shown that it is more efficient than the liquid form (Sauer et al. 2008). It was believed that D-methionine is 90% converted to L-methionine and that DL-methionine is 95% bioavailable (Baker, 1994) although there were clear data about DL-methionine being excreted at 10% and being oxidized at 5.5 % (Saunderson, 1985 and 1987). Saunderson works did not get enough attention because L-methionine was not commercially available. Nevertheless, L-methionine became commercially available in 2014 as the only natural source of methionine. L-methionine can be directly utilized for growth and physiological needs (a game changer for the industry). L-methionine is available in high volumes
produced from renewable resources. Crystalline L-methionine provides the opportunity of relieving farm animals from the unnecessary job to convert the isomers and precursors of methionine to L-methionine.
D-amino acids exists in plants up to 8% of the corresponding L-forms
It is a widely accepted fact that plants harbor free D-AAs as they could be identified in different plant species and tissues (Kolukisaoglu and Suarez, 2017). Free D-AAs in the range of about 0.2% up to 8% relative to the corresponding L-AAs were found in plants (Bruckner and Westhauser, 2003).
Processing (heating, alkaline, and acid treatment) increases the content of D-amino acids
Processing increases the content of D-AAs (Caspo et al. 2009). Some insects, worms and marine invertebrates also contain substantial
quantities of D-AAs (Caspo et al. 2009), for example, quantities of D-AAs occurring in marine shellfish can e xceed 1%. On examination of free D-AAs in milk, fermented milk, fresh cheese and curd cheese, Bruckner and Hausch (1990) established that considerable quantities of D-AAs occur both in raw milk and in fermented dairy products manufactured from it. The empirical data obtained by the above authors are presented in Table 1.
Feed industry uses a mixture of different processing technologies such as pelleting, extrusion, pressure, etc. Similar to the food industry. Table 2 gives the D-AA content of various processed foodstuffs in comparison with untreated controls. Heat treatment or combined heat and alkali treatment in every case gave rise to D-AAs in measurable quantities.
The highest D-aspartic acid content (31%) was determined in the casein heated to 230 °C for 20 minutes.
Plants contain D-form of AAs and feed processing makes the content of D-AAs even higher. Thus, there is a hidden pressure on the enzymatic machinery which are supposed to convert D-AAs to the L-form. The question is why one should exert an additional pressure to the cellular system of farm animals in order to convert D-methionine to L-methionine. Farm animals already have a high metabolic rate because of their fast growth. Nutritionists can reduce the stress on the farm animals by means of using a commercially available L-methionine.
To answer the question at the beginning of this article: farm animals have to convert not only D-methionine to the L-form but also other D-AAs which does exists in the plants or the ones in which their concentrations are increased because of processing the raw materials or complete feed.
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2. Brü ckner, H. and M. Hausch. 1990. D-amino acids in dairy products: Detection, origin and nutritional aspects. I. Milk, fermented milk, fresh cheese and acid curd cheese. Milchwissenschaft. 45: 357-360.
3. Brü ckner, H. and T. Westhauser. 2003. Chromatographic determination of L- and D-amino acids in plants. Amino Acids. 24:4355.
4. Csapo, J. , Albert, Cs. and Zs. Csapo-Kiss. 2009. The D-amino acid content of foodstuffs (A Review). Acta Univ. Sapientiae, Alimentaria, 2(1) 5-30.
5. Kolukisaoglu, ü . and J. Suarez. 2017. D-Amino Acids in Plants: New Insights and Aspects, but also More Open Questions. doi.org/10.5772/intechopen.68539.
6. Sauer, N., K. Emrich, H.-P. Piepho, A. Lemme, M. S. Redshaw, and R. Mosenthin. 2008. Meta-Analysis of the Relative Efficiency of Methionine-Hydroxy-Analogue-Free-Acid Compared with dl-Methionine in Broilers Using Nonlinear Mixed Models. Poultry Science 87:20232031.
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8. Saunderson, C.L. 1987. Effect of fasting and of methionine deficiency on L-Methionine, DL-Methionine and DL-2-hydroxy-4-methylthiobutanoic acid metabolism in broiler chicks. Br. J. Nutr. 57:429-437.