Specificity of the imino acid carrier in rat small intestine.

The rat intestinal imino acid carrier is chloride independent, while in guinea pig and rabbit intestine it is chloride dependent. While non-alpha-amino acids do not significantly interact with guinea pig and rabbit imino acid carriers, inhibition studies had indicated that in rat small intestine beta-alanine, gamma-aminobutyric acid (GABA), and probably taurine might be transported by the imino acid carrier. The present study of rat jejunum demonstrates that the half-maximal activation concentration of beta-alanine (K1/2 beta-Ala) is identical to its inhibition constant (Ki beta-Ala) against GABA, that K1/2GABA is identical to KiGABA against beta-alanine, that proline and sarcosine have identical values of Ki against beta-alanine and GABA, and that Ki of beta-alanine and proline against sarcosine are equal to their K1/2 values. Taurine inhibits the transport of beta-alanine, and 300 mM proline and beta-alanine reduce the transport of taurine measured at 80 mM taurine to the level expected for the diffusive contribution, corresponding to Ki values equal to those against sarcosine. Thus the rat imino acid carrier is the principal carrier of taurine and the only carrier of beta-alanine and GABA. It is also demonstrated that alpha-amino-monocarboxylic acids with side chains in excess of one methyl group do not significantly interact with the imino acid carrier, and the lack of stereospecificity is confirmed.