Shennan D B, Calvert D T, Backwell F R, Boyd C A
Hannah Research Institute, Ayr KA6 5HL, UK.
Biochim Biophys Acta. 1998 Aug 14;1373(1):252-60. doi: 10.1016/s0005-2736(98)00107-2.
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 uptake of the constituent amino acids or were taken up intact. In view of this, we have designed experiments to investigate (a) whether the lactating rat mammary gland is capable of transporting hydrolysis-resistant dipeptides and (b) whether or not mammary cells are able to hydrolyse peptides, including glutathione, extracellularly. The uptake of the hydrolysis-resistant dipeptides D-[3H]Phe-L-Gln and D-[3H]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 dipeptide suggesting that the small, albeit significant, uptake of the dipeptides 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-Glu were able to trans-accelerate the efflux of labelled D-aspartate from preloaded rat mammary tissue (explants and perfused mammary gland). It appears that these peptides were being hydrolysed extracellularly followed by the uptake of free anionic amino acids via the mammary tissue high affinity, Na+-dependent anionic amino acid carrier operating in the exchange mode. Glutathione was able to trans-accelerate D-aspartate efflux from lactating rat mammary tissue in a fashion which was sensitive to the peptidase inhibitor acivicin. This suggests that gamma-glutamyltranspeptidase hydrolyses glutathione to produce L-glutamate which is subsequently transported via the high-affinity anionic amino acid carrier. Hydrolysis of peptides followed by uptake of the constituent amino acids may provide an important source of amino acids for milk protein synthesis.
最近的研究表明，哺乳期乳腺能够利用血浆来源的二肽进行乳蛋白合成。然而，尚不清楚这些肽是先被水解，随后其组成氨基酸被摄取，还是完整地被摄取。鉴于此，我们设计了实验来研究：（a）哺乳期大鼠乳腺是否能够转运抗水解二肽；（b）乳腺细胞是否能够在细胞外水解包括谷胱甘肽在内的肽。灌注的大鼠乳腺对抗水解二肽D-[³H]苯丙氨酸-L-谷氨酰胺和D-[³H]苯丙氨酸-L-谷氨酸的摄取量较低。同时添加L-亮氨酸-L-丙氨酸（50 mM）对两种标记二肽的清除均无影响，这表明二肽虽有少量但显著的摄取并非通过高亲和力肽转运体（PepT1/PepT2）。除D-苯丙氨酸-L-谷氨酸外，所有测试的阴离子二肽（L-谷氨酸-L-丙氨酸、L-天冬氨酸-L-丙氨酸、L-丙氨酸-L-天冬氨酸、L-天冬氨酸-甘氨酸、甘氨酸-L-天冬氨酸和甘氨酸-L-谷氨酸）均能加速预加载的大鼠乳腺组织（外植体和灌注乳腺）中标记的D-天冬氨酸的流出。看来这些肽在细胞外被水解，随后游离阴离子氨基酸通过以交换模式运作的乳腺组织高亲和力、Na⁺依赖性阴离子氨基酸载体被摄取。谷胱甘肽能够以对肽酶抑制剂阿西维辛敏感的方式加速哺乳期大鼠乳腺组织中D-天冬氨酸的流出。这表明γ-谷氨酰转肽酶将谷胱甘肽水解产生L-谷氨酸，随后L-谷氨酸通过高亲和力阴离子氨基酸载体转运。肽被水解后其组成氨基酸被摄取可能为乳蛋白合成提供重要的氨基酸来源。
