Identification of a candidate membrane protein for the basolateral peptide transporter of rat small intestine.

A candidate protein for the basolateral peptide transporter of rat jejunum is described. Vascular perfusion of the photoaffinity label, [4-azido-D-phe]-L-ala (2.5mM), had no effect on the transepithelial transport of the non-hydrolysable dipeptide D-phe-L-gln (1mM) from the lumen, its mucosal accumulation or wash-out into the vascular perfusate. When the label was perfused luminally, the transepithelial transport of D-phe-L-gln was inhibited by 38% (P<0.001) and accumulation increased by 62% (P<0.05). These data are consistent with those of a basolateral transporter that is strongly asymmetric in its substrate binding and transport properties. Labelling of basolateral membrane vesicles with [4-azido-3,5-3H-D-phe]-L-ala revealed that the majority of label was incorporated into a single protein of M(r)112+/-2 kDa and pI 6.5. MALDI-TOF analysis of tryptic digests of the protein followed by database searches established that this protein was novel with no obvious similarity to PepT1, the apical membrane transporter.