Romani Ana Paula, Ito Amando Siuiti
Departamento de Física e Matemática, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Brazil.
Biophys Chem. 2009 Feb;139(2-3):92-8. doi: 10.1016/j.bpc.2008.10.009. Epub 2008 Nov 6.
Adrenocorticotropin (ACTH) and alpha-melanocyte stimulating hormone (alpha-MSH) are peptides which present many physiological effects related to pigmentation, motor and sexual behavior, learning and memory, analgesia, anti-inflammatory and antipyretic processes. The 13 amino acid residues of alpha-MSH are the same initial sequence of ACTH and due to the presence of a tryptophan residue in position 9 of the peptide chain, fluorescence techniques could be used to investigate the conformational properties of the hormones in different environments and the mechanisms of interaction with biomimetic systems like sodium dodecyl sulphate (SDS) micelles, sodium dodecyl sulphate-poly(ethylene oxide) (SDS-PEO) aggregates and neutral polymeric micelles. In buffer solution, fluorescence parameters were typical of peptides containing tryptophan exposed to the aqueous medium and upon addition of surfactant and polymer molecules, the gradual change of those parameters demonstrated the interaction of the peptides with the microheterogeneous systems. From time-resolved experiments it was shown that the interaction proceeded with conformational changes in both peptides, and further information was obtained from quenching of Trp fluorescence by a family of N-alkylpyridinium ions, which possess affinity to the microheterogeneous systems dependent on the length of the alkyl chain. The quenching of Trp fluorescence was enhanced in the presence of charged micelles, compared to the buffer solution and the accessibility of the fluorophore to the quencher was dependent on the peptide and the alkylpyridinium: in ACTH(1-21) highest collisional constants were obtained using ethylpyridinium as quencher, indicating a location of the residue in the surface of the micelle, while in alpha-MSH the best quencher was hexylpyridinium, indicating insertion of the residue into the non-polar region of the micelles. The results had shown that the interaction between the peptides and the biomimetic systems where driven by combined electrostatic and hydrophobic effects: in ACTH(1-24) the electrostatic interaction between highly positively charged C-terminal and negatively charged surface of micelles and aggregates predominates over hydrophobic interactions involving residues in the central region of the peptide; in alpha-MSH, which presents one residual positive charge, the hydrophobic interactions are relevant to position the Trp residue in the non-polar region of the microheterogeneous systems.
促肾上腺皮质激素（ACTH）和α-黑素细胞刺激素（α-MSH）是具有多种生理作用的肽类，这些作用与色素沉着、运动和性行为、学习与记忆、镇痛、抗炎和退热过程相关。α-MSH的13个氨基酸残基与ACTH的起始序列相同，并且由于肽链第9位存在色氨酸残基，荧光技术可用于研究激素在不同环境中的构象性质以及与十二烷基硫酸钠（SDS）胶束、十二烷基硫酸钠 - 聚环氧乙烷（SDS - PEO）聚集体和中性聚合物胶束等仿生系统的相互作用机制。在缓冲溶液中，荧光参数是含色氨酸且暴露于水介质中的肽的典型特征，加入表面活性剂和聚合物分子后，这些参数的逐渐变化表明肽与微非均相系统发生了相互作用。从时间分辨实验可知，相互作用过程中两种肽都发生了构象变化，并且通过一类对微非均相系统具有亲和力且依赖于烷基链长度的N - 烷基吡啶鎓离子对色氨酸荧光的猝灭获得了更多信息。与缓冲溶液相比，在带电荷胶束存在下色氨酸荧光的猝灭增强，并且荧光团对猝灭剂的可及性取决于肽和烷基吡啶鎓：在ACTH(1 - 21)中，使用乙基吡啶鎓作为猝灭剂时获得最高的碰撞常数，表明该残基位于胶束表面，而在α-MSH中最佳猝灭剂是己基吡啶鎓，表明该残基插入到胶束的非极性区域。结果表明，肽与仿生系统之间的相互作用是由静电和疏水作用共同驱动的：在ACTH(1 - 24)中，带高度正电荷的C末端与胶束和聚集体带负电荷表面之间的静电相互作用比涉及肽中心区域残基的疏水相互作用占主导；在具有一个残余正电荷的α-MSH中，疏水相互作用对于将色氨酸残基定位在微非均相系统的非极性区域至关重要。
