Influx of neutral amino acids across the brush border of rabbit ileum: Stereospecificity and the roles of the α-amino and α-carboxylate groups

Abstract

The influxes of the l- and d-stereoisomers of alanine, valine, serine, leucine, histidine, phenylalanine and tryptophan across the brush border of rabbit ileum and the roles of the α-carboxylate and α-amino groups in the influx process have been examined. Our results indicate that: 1. 1. The interactions between neutral amino acids and the influx mechanism(s) invovle the α-amino and α-carboxylate groups as well as the side chain. 2. 2. The requirement for both the α-amino and α-carboxylate groups indicate that their interactions with the influx mechanism are cooperative rather than independent. 3. 3. With the exception of d-valine, the influxes of all of the d-amino acids examined display saturation kinetics, are Na +-dependent and are subject to classical competitive inhibition by their l-enantiomorphs. The influx of d-valine appears to be largely attributable to simple diffusion, although a small mediated component characterized by a high K 1 cannot be excluded. 4. 4. When d-amino acids interact with their influx mechanisms, the neutral side chain is displaced by 180 degrees from the “natural” position, and the ability of the d-amino acids to interact with their influx mechanisms appears to depend upon the ability of the displaced side chain to engage in interactions with neighboring groups.