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Article

Protein-Bound D-Amino Acids, and to a Lesser Extent Lysinoalanine, Decrease True Ileal Protein Digestibility in Minipigs as Determined with ¹⁵N-Labeling^{1 ,2}

Federal Dairy Research Centre, Department of Physiology and Biochemistry of Nutrition, D-24103 Kiel, Germany and ^* Research Institute for the Biology of Farm Animals, Division of Nutritional Physiology, D-18059 Rostock, Germany

³To whom correspondence should be addressed.

Heat and alkali treatment of food may increase the concentrations of protein-bound D-amino acids and cross-links such as lysinoalanine (LAL). To examine how protein treatment affects digestibility, purified test meals [total protein 150 g/kg dry matter (DM), 0.44 MJ/(kg BW^0.75 · d)] were prepared, containing (g/kg DM) casein, 75; ß-lactoglobulin, 50; or wheat protein, 40. Each was ¹⁵N-labeled. Test proteins were used either in their native form or after treatment for 6 or 24 h at 65°C, pH 10.5–11.5. Each meal was fed to nine adult miniature pigs (twofold complete cross-classification). Chyme was collected continuously over 33 h postprandially via T-fistulas in the terminal ileum, and digestibilities of test proteins and individual L- and D-amino acids were calculated on the basis of recovery of ¹⁵N and the respective amino acids in the chyme. Treatment of casein, ß-lactoglobulin or wheat protein for 24 h increased levels of D-amino acid residues. L-Asparagine and aspartate (L-Asx) were particularly susceptible; 14.7 ± 0.4, 11.7 ± 0.2 and 11.0 ± 0.9%, respectively, underwent racemization. LAL levels increased in parallel; 11.7 ± 0.3, 13.6 ± 0 and 14.8 ± 0.0%, respectively, of total lysine was converted to LAL. At the same time, prececal protein digestibility was decreased by 13.4 ± 2.3, 15.3 ± 1.4 and 17.8 ± 1.2% units, respectively (P < 0.05; mean ± SEM, n = 9). Digestibility of individual L-amino acids decreased by 10–15%, but L-amino acids prone to peptic cleavage, such as L-phenylalanine and L-tyrosine, were not affected. Digestibilities of D-amino acids and LAL were ~35%. It seems that mainly D-amino acids, and to a lesser extent LAL, were responsible for lower digestibility by interfering with peptic cleavage.

KEY WORDS: • miniature pigs • true protein digestibility • D-amino acids • lysinoalanine • ¹⁵N-labeling

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