Protein-bound D-amino acids, and to a lesser extent lysinoalanine, decrease true ileal protein digestibility in minipigs as determined with N- labelling

Abstract

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 BW0.75.d)] were prepared, containing (g/kg DM) casein, 75; β-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, β-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.