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 (K-1/2(beta-Ala)) is identical to its inhibition constant (K-i(beta-Ala)) against GABA, that K-1/2(GABA) is identical to K-i(GABA) against beta-alanine, that proline and sarcosine have identical values of K-i against beta-alanine and GABA, and that K-i of beta-alanine and proline against sarcosine are equal to their K-1/2 values. Taurine inhibits the transport of beta-alanine, and 300 mM proline and alpha-alanine reduce the transport of taurine measured at 80 mM taurine to the level expected for the diffusive contribution, corresponding to K-i 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.