Elucidation of intestinal absorption of d, l-amino acid enantiomers and aging in rats

Abstract

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 free d, l-AA enantiomers (arginine, alanine and aspartic acid as models for basic, neutral and acidic AAs, respectively, in this study) and the relationship between age and absorption were investigated. The degree of rat intestinal absorption of free d, l-AAs was evaluated using apparent membrane permeability 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 enantiomers in perfusion (in- and outflow) solutions were carried out by the in-capillary derivatization high-performance capillary electrophoretic methods (ICD-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 weeks 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’.