TRIPEPTIDE TRANSPORT IN RAT LUNG

Transport Of L-alanyl-D-phenylalanyl-L-alanine was investigated with a n in situ vascular perfusion preparation of rat lung and brush border membrane vesicles prepared from type II pneumocytes. In the perfused l ung 1 mM tripeptide was transported intact from the alveolar lumen to the vascular perfusate at a mean rate of 25.1 +/- 1.29 (3) nmol/min pe r g dry weight. D-Phenylalanine also appeared in the vascular perfusat e at a rate of 21.9 +/- 1.74 (3) nmol/min per g dry weight indicating that 47% of the absorbed tripeptide was split during passage across th e epithelial layer. No dipeptide could be detected in the vascular eff luent during perfusions with tripeptide. Rapid L-alanyl-D-phenylalanyl -L-alanine uptake occurred with fresh apical membrane vesicles prepare d from type II pneumocytes and this was abolished by treatment with 0. 1% triton. The related tripeptide, D-alanyl-L-phenylalanyl-D-alanine, was taken up significantly more slowly by the vesicles. D-phenylalanyl -L-alanine and D-phenylalanyl-D-alanine, were also studied with the va scularly perfused preparation; the mixed dipeptide appeared in the vas cular perfusate significantly faster than L-alanyl-D-phenylalanyl-L-al anine whereas D-phenylalanyl-D-alanine appeared more slowly and was no t hydrolysed.