Lengyel I, Toth F, Biyashev D, Szatmari I, Monory K, Tomboly C, Toth G, Benyhe S, Borsodi A
Institute of Biochemistry, Biological Research Center, Hungarian Academy of Sciences, Szeged, Hungary.
present address: UCL Institute of Ophthalmology, University College London, London, United Kingdom.
J Physiol Pharmacol. 2016 Aug;67(4):605-616.
Endomorphins are natural amidated opioid tetrapeptides with the following structure: Tyr-Pro-Trp-Phe-NH2 (endomorphin-1), and Tyr-Pro-Phe-Phe-NH2 (endomorphin-2). Endomorphins interact selectively with the μ-opioid or MOP receptors and exhibit nanomolar or sub-nanomolar receptor binding affinities, therefore they suggested to be endogenous agonists for the μ-opioid receptors. Endomorphins mediate a number of characteristic opioid effects, such as antinociception, however there are several physiological functions in which endomorphins appear to act in a fashion that does not involve binding to and activation of the μ-opioid receptor. Our recent data indicate that a radiolabelled [H]endomorphin-1 with a specific radioactivity of 2.35 TBq/mmol - prepared by catalytic dehalogenation of the diiodinated peptide precursor in the presence of tritium gas - is able to bind to a second, naloxone insensitive recognition site in rat brain membranes. Binding heterogeneity, i.e., the presence of higher (K = 0.4 nM / B = 120 fmol/mg protein) and lower (K = 8.2 nM / B = 432 fmol/mg protein) affinity binding components is observed both in saturation binding experiments followed by Schatchard analysis, and in equilibrium competition binding studies. The signs of receptor multiplicity, e.g., curvilinear Schatchard plots or biphasic displacement curves are seen only if the non-specific binding is measured in the presence of excess unlabeled endomorphin-1 and not in the presence of excess unlabeled naloxone. The second, lower affinity non-opioid binding site is not recognized by heterocyclic opioid alkaloid ligands, neither agonists such as morphine, nor antagonists such as naloxone. On the contrary, endomorphin-1 is displaced from its lower affinity, higher capacity binding site by several natural neuropeptides, including methionine-enkephalin-Arg-Phe, nociceptin-orphanin FQ, angiotensin and FMRF-amide. This naloxone-insensitive, consequently non-opioid binding site seems to be present in nervous tissues carrying low density or no μ-opioid receptors, such as rodent cerebellum, or brain of μ-opioid receptor deficient (MOPr) transgenic or 'knock-out' (K.O.) mice. The newly described non-opioid binding component is not coupled to regulatory G-proteins, nor does it affect adenylyl cyclase enzyme activity. Taken together endomorphin-1 carries opioid and, in addition to non-opioid functions that needs to be taken into account when various effects of endomorphin-1 are evaluated in physiological or pathologic conditions.
酪氨酰-脯氨酰-色氨酰-苯丙氨酰胺（内吗啡肽-1）和酪氨酰-脯氨酰-苯丙氨酰-苯丙氨酰胺（内吗啡肽-2）。内吗啡肽与μ-阿片受体或MOP受体选择性相互作用，并表现出纳摩尔或亚纳摩尔的受体结合亲和力，因此它们被认为是μ-阿片受体的内源性激动剂。内吗啡肽介导多种典型的阿片样物质效应，如镇痛作用，然而在一些生理功能中，内吗啡肽似乎以一种不涉及与μ-阿片受体结合和激活的方式发挥作用。我们最近的数据表明，通过在氚气存在下对二碘化肽前体进行催化脱卤制备的比活度为2.35 TBq/mmol的放射性标记[H]内吗啡肽-1能够与大鼠脑膜中的第二个对纳洛酮不敏感的识别位点结合。在饱和结合实验后进行Scatchard分析以及平衡竞争结合研究中均观察到结合异质性，即存在较高亲和力（K = 0.4 nM / B = 120 fmol/mg蛋白质）和较低亲和力（K = 8.2 nM / B = 432 fmol/mg蛋白质）的结合成分。仅当在过量未标记的内吗啡肽-1存在下而非过量未标记的纳洛酮存在下测量非特异性结合时，才会出现受体多样性的迹象，例如曲线型Scatchard图或双相置换曲线。第二个较低亲和力的非阿片样物质结合位点不被杂环阿片样生物碱配体识别，既不被吗啡等激动剂识别，也不被纳洛酮等拮抗剂识别。相反，包括甲硫氨酸脑啡肽-精氨酰-苯丙氨酸、孤啡肽、血管紧张素和FMRF酰胺在内的几种天然神经肽可将内吗啡肽-1从其较低亲和力、较高容量的结合位点上置换下来。这个对纳洛酮不敏感、因此是非阿片样物质的结合位点似乎存在于低密度或无μ-阿片受体的神经组织中，如啮齿动物小脑或μ-阿片受体缺陷（MOPr）转基因或“敲除”（K.O.）小鼠的大脑。新描述的非阿片样物质结合成分不与调节性G蛋白偶联，也不影响腺苷酸环化酶的酶活性。综上所述，内吗啡肽-1具有阿片样物质功能，此外还具有非阿片样物质功能，在生理或病理条件下评估内吗啡肽-1的各种效应时需要考虑这些功能。
