Comparison of different dietary amino acids as inducers of intestinal amino acid transport

Abstract

Intestinal amino acid (AA) transporters are known to be induced by raised levels of dietary protein or free AA mixtures, but little is known about whether certain AAs are more potent inducers than others. Hence we compared, in mouse jejunum, the inductive effects of seven different AAs (given to mice as dietary supplements) on the brush-border uptake of five solutes predominantly taken up via different transporters. The AAs tested as dietary supplements included one imino acid, two acidic AAs, two basic AAs, and two neutral AAs. The solutes whose uptakes we studied were D-glucose plus preferential substrates for the acidic AA, basic AA, neutral AA, and imino acid transporter. Mouse growth rates were consistently higher for dietary supplementation with nonessential than essential AAs, but there were no ration-related differences in intestinal morphometric parameters and almost none in active D-glucose uptake. AA transport is regulated independently of D-glucose transport. The four AA transporters are regulated semi-independently on each other: e.g., aspartate is a good inducer for only two of the four transporters, arginine for one or two, lysine for just one. Different AAs differ in their potencies at inducing the same AA transporter. In a few cases good substrates make good inducers (e.g., aspartate of the acidic AA transporter, valine of the neutral AA transporter). But often this is not true: the acidic AA aspartate is the best inducer of the basic AA transporter; the basic AA arginine but not lysine is a good inducer of the acidic AA transporter; and the imino acid proline is not a good inducer of the imino acid transporter. The adaptive significance of these discrepancies between inducers and transported substrates is unclear. Thus the differing dietary values of different AAs may be related to their differing values as inducers of AA transport, in addition to their well-known differing biosynthetic values as essential or nonessential nutrients.