Structure-Affinity Relationships of Substrates for the Neutral Amino Acid Transport System in Rabbit Ileum

The apparent affinities of various amino acids for the neutral amino acid transport system in rabbit ileum were determined by measuring the inhibition of L-methionine-¹⁴C influx across the brush border membrane. The apparent affinity was very low for compounds lacking an α-amino group, compounds with the α-hydrogen substituted by a methyl group, D-compounds, compounds with tertiary branching in the side chain, compounds with either a positive or negative charge in the side chain, and in most cases, compounds with a hydrophilic moiety in the side chain. High apparent affinities were exhibited by compounds with unbranched carbon or carbon-sulfur side chains. Branched compounds such as valine and leucine exhibited affinities which correlate with binding of only the linear portion of the side chain. The calculated change in free energy of binding is 370 cal/mol/CH₂ group which suggests the binding region for the side chain is partially hydrophobic. The affinities of families of analogues, derivatives of cysteine, methionine, serine, alanine, valine, and phenylalanine, correlate with their calculated octanol/water partition coefficients and are also correlated with apparent structural and electronic differences between families. The data permit a preliminary description of the functional geometry of the neutral amino acid transport site. The site contains a region for binding the α-amino group, α-carboxyl group, and side chain. The regions about the α-amino group and α-hydrogen are quite sterically limited. The side chain binding region is hydrophobic in nature and appears to be shallow, binding only the linear portion of branched or ring compounds.