Structural Requirements for the Intestinal Mucosal-Cell Peptide Transporter: The Need for N-Terminal α-Amino Group

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Abstract
The requirement for a free α-amino group for the intestinal peptide carrier-mediated transport was investigated. A series of dipeptide analogues without the N-terminal α-amino group [including phenyl-propionylproline, phenylacetylproline, N-benzoylproline, phenylacetyl-α-methyldopa, and hippuric acid (N-benzoylglycine)] were studied in the perfused rat intestinal segment. The absorption of phenylpropionylproline, phenylacetyl-α-methyldopa, and N-benzoylproline was concentration dependent. The transport parameters (mean ± SD) of phenylpropionylproline and N-benzoylproline were as follows: Jmax*, 0.037 (±0.019) mM; Km, 0.045 (±0.027) mM; Pc*, 0.830 (±0.130); and Pm*, 0.673 ± 0.049; and Jmax*, 1.34 (±0.24) mM; Km, 1.31 (±0.30) mM; Pc*, 1.02 (±0.11); and Pm* 0; respectively. The intestinal permeabilities of phenylpropionylproline, phenylacetylproline, N-benzoylproline, and hippuric acid (N-benzoylglycine) were significantly reduced by dipeptides and ceph-radine. These results strongly suggest that these dipeptide analogues, without an α-amino group, are transported by the peptide carrier and provide more direct evidence that a free α-amino group is not absolutely essential for the mucosal-cell peptide carrier-mediated transport.

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