Elucidation of intestinal absorption of D,L-amino acid enantiomers and aging in rats

At the present time, the origin of protein bound D-amino acid (AA) has been fairly well elucidated, but that of free D-AA is still not well understood . To gain greater understanding of this, intestinal absorption in rats of f ree D,L-AA enantiomers (arginine, alanine and aspartic acid as models for b asic, neutral and acidic AAs, respectively, in this study) and the relation ship between age and absorption were investigated. The degree of rat intest inal absorption of free D,L-AAS was evaluated using apparent membrane perme ability coefficients (P-app) which were obtained from an in situ intestinal single-pass perfusion method with Krebs-Ringer bicarbonate buffer (pH 7.4) solution containing D,L-AA enantiomers. Determinations of D,L-AA enantiome rs in perfusion (in- and outflow) solutions were carried out by the in-capi llary derivatization high-performance capillary electrophoretic methods (IC D-HPCE methods) that were previously developed by our group. Collectively, our observations suggest: (1) that the P-app Of L-AA is higher than that of the D-isomer; (2) that D-AA can be absorbed as well as L-AA using a sodium ion-dependent transporter that is located on the brush border membrane of rat intestinal epithelial cells; (3) that P-app reached a maximum at 8 week s of age, but were measured at decreased amounts at 52 and 104 weeks of age . These results suggest that free D-AA in a mammalian body originates from 'exogenous sources'. (C) 1999 Elsevier Science B.V. All rights reserved.