Studies Opioid Antagonist Peptides with Retro-Orientation

我们得到了许多氨基末端含有Tyr-Och_3的阿片拮抗肽。对于这些拮抗肽，假设了一个逆定向模型，在该模型中，多肽与阿片受体的结合方向与激动肽的方向相反。类阿片拮抗肽，如酪蛋白6(SerArg-Tyr-Tyr-Pro-Ser-Tyr-Och_3)、酪蛋白5(Arg-Tyr-Pro-Ser-Tyr-Och_3)、酪蛋白4(Tyr-Pro-Ser-Tyr-Och_3)、乳铁蛋白A(Tyr-Leu-Gly-Ser-Gly-Tyr-Och_3)、乳铁蛋白B(Arg-Tyr-Tyr-Gly-Tyr-Och_3)、乳铁蛋白C(Lys-Tyr-Leu-Gly-Gly-Pro-GLN-Tyr-Och_3)、反式脑啡肽甲酯和反式强啡肽A[1-13]甲酯符合该模型。这些多肽以通式X_a-芳香-X_b-酪氨酸-X_c表达，当X_a为碱性残基时，拮抗肽与kappa亚型阿片受体具有亲和力。这与阿片激动肽以通式X_a-Tyr-X_b-芳香-X_c表达形成鲜明对比，当X_c为碱性残基时，这些多肽对kappa亚型阿片受体具有选择性。在这样的逆定向模型中，Tyr残基的氨基没有投射到受体上的适当位置。因此，这些多肽虽然与阿片受体有亲和力，但不具有激动剂活性，也得到了其他不符合逆定向模型的阿片拮抗剂多肽。例如，酪毒素A(Tyr-Pro-Ser-Tyr-Gly-Leu-Asn-Tyr)、酪蛋白C(Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg)都来自牛酪蛋白，而来自人的酪蛋白D(Tyr-Val-Pro-Phe-Pro-Phe)不符合反向取向模型。阿片类拮抗剂的类型似乎很多，因为拮抗剂与受体的结合方式比拮抗剂的结合方式更具多样性。

We obtained many opioid antagonist peptides which have Tyr-OCH_3 at their amino termini. For theses antagonist peptides, a retro-orientation model, in which the peptides bind to the opioid receptors just in a reverse orientation to that of the agonist peptides, was postulated. Opioid antagonist peptides such as casoxin 6 (Ser-Arg-Tyr-Pro-Ser-Tyr-OCH_3), casoxin 5 (Arg-Tyr-Pro-Ser-Tyr-OCH_3), casoxin 4 (Tyr-Pro-Ser-Tyr-OCH_3), lactoferroxin A (Tyr-Leu-Gly-Ser-Gly-Tyr-OCH_3), lactoferroxin B (Arg-Tyr-Tyr-Gly-Tyr-OCH_3), lactoferroxin C (Lys-Tyr-Leu-Gly-Pro-Gln-Tyr-OCH_3), retro-[Leu] enkephalin methylester and retro-dynorphin A [1-13] methylester fit to this model. These peptides are expressed in a general formula, X_a-Aromatic-X_b-Tyr-X_c. The antagonist peptides had affinity for kappa-subtype opioid receptor when X_a were basic residues. This is a good contrast to that opioid agonist peptides are expressed in a genaral formula, X_a-Tyr-X_b-Aromatic-X_c, and that the peptides show selectivity for kappa-subtype opioid receptors when X_c are basic residues. In such a retro-orientation model, amino group of Tyr residue is not projected to a proper site on the receptors. For this reason, the peptides could not have the agonist activity in spite of the affinities to the opioid receptors.Other opioid antagonist peptides which do not fit to the retro-orientation model were also obtained. For example, casoxin A (Tyr-Pro-Ser-Tyr-Gly-Leu-Asn-Tyr), casoxin C (Tyr-Ile-Pro-Ile-Gln-Tyr-Val-Leu-Ser-Arg) both derived bovine kappa-casein, and casoxin D (Tyr-Val-Pro-Phe-Pro-Pro-Phe) derived from human alpha_<s1>-casein do not fit to the retro-orientation model. There seems to be many types of opioid antagonists because manner of the binding between the antagonists and receptors are more variable than that of the antagonists.