PEPTIDE AMINONITROGEN TRANSPORT BY THE LACTATING RAT MAMMARY-GLAND

Recent studies have shown that the lactating mammary gland is able to utilize plasma-derived dipeptides for milk protein synthesis. However, it was not clear whether the peptides were hydrolysed followed by upt ake of the constituent amino acids or were taken up intact. In view of this, we have designed experiments to investigate (a) whether the lac tating rat mammary gland is capable of transporting hydrolysis-resista nt dipeptides and (b) whether or not mammary cells are able to hydroly se peptides, including glutathione, extracellularly. The uptake of the hydrolysis-resistant dipeptides D-[H-3]Phe-L-Gln and D-[H-3]Phe-L-Glu by the perfused rat mammary gland was low. Concomitant addition of L- Leu-L-Ala (50 mM) had no effect on the clearance of either labelled di peptide suggesting that the small, albeit significant, uptake of the d ipeptides is not via a high affinity peptide transporter (PepT1/PepT2) . All anionic dipeptides tested (L-Glu-L-Ala, L-Asp-L-Ala, L-Ala-L-Asp , L-Asp-Gly, Gly-L-Asp and Gly-L-Glu) with the exception of D-Phe-L-Gl u were able to trans-accelerate the efflux of labelled D-aspartate fro m preloaded rat mammary tissue (explants and perfused mammary gland). It appears that these peptides were being hydrolysed extracellularly f ollowed by the uptake of free anionic amino acids via the mammary tiss ue high affinity, Na+-dependent anionic amino acid carrier operating i n the exchange mode. Glutathione was able to trans-accelerate D-aspart ate efflux from lactating rat mammary tissue in a fashion which was se nsitive to the peptidase inhibitor acivicin. This suggests that gamma- glutamyltranspeptidase hydrolyses glutathione to produce L-glutamate w hich is subsequently transported via the high-affinity anionic amino a cid carrier. Hydrolysis of peptides followed by uptake of the constitu ent amino acids may provide an important source of amino acids for mil k protein synthesis. (C) 1998 Elsevier Science B.V. All rights reserve d.