Structures And Biological Activity Of Alkaloids And Othe

两栖类皮肤提供了广泛的生物活性生物碱，其中许多具有独特的药理活性和治疗潜力。这些生物碱包括有效的钠通道激活剂Batrachooxin，非竞争性烟碱受体通道阻滞剂的组氨酸毒素，pumiliotoxins/allopumiliotoxins/homopumiliotoxins及其相关的同系物，其中一些由于对钠通道的作用而具有强肌和强心活性，epbatidine是一种非常有效和选择性的尼古丁激动剂，具有强大的抗伤害性活性，表喹酰胺是一种对某些受体亚型具有选择性的烟碱激动剂。其他生物碱包括十氢喹啉类、吡咯利齐定类、吲哚里西定类、喹唑类、来咪嗪类，以及各种三环生物碱，包括螺吡咯里西定肟、地理毒素、伪菲那明类、环戊喹啉类、球藻碱类和球衣草碱类似物。在某些有毒禽类和食用虫中也发现了巴曲霉毒素。有机化合物的结构鉴定现在几乎完全基于光谱分析，使用紫外线(UV)、红外(IR)、质量(MS)和核磁共振(NMR)光谱技术。我们的天然产品计划依赖于强大的光谱技术的开发，用于分析从两栖动物皮肤和其他来源的提取物中获得的复杂混合物中微量存在的生物碱和其他化合物。关键技术是气相色谱(GC)或高效液相色谱(HPLC)分离，然后对UV、IR和MS数据进行在线分析。这些技术，加上微量化学反应的发展，包括氢化、酰化、顺式二醇的丁基硼化和用甲醛进行的GC分析中的还原N-甲基化，已经对青蛙皮提取物中的800多种生物碱进行了详细的表征，代表了大约26个结构类别。高效液相色谱-质谱法可以研究所有生物碱，即使是那些高分子量或高极性的生物碱，但由于缺乏大气压化学电离(APCI)或电喷雾电离(ESI)的广泛碎裂，只能提供有限的结构见解。使用电子碰撞电离(EIMS)的GC-MS分析提供了丰富的、可诊断的碎裂模式，而化学电离(CIMS)则提供了分子量和可交换的OH和NH基团的数量。这种开创性的光谱研究已经扩展到在碰撞激活的CIMS模式下开发和应用串联质谱仪，展示和阐明不同于传统EIMS的碎片，以及对传统EIMS的补充。气相GC-FTIR(傅立叶变换红外光谱)的分析潜力使其能够从传统的IR用途(鉴定官能团，如羟基、羰基、双键和三键等)扩展到提供有价值的立体化学见解(顺环或跨环连接，Bohlmann带分析，以指示氢在氮附近碳上的取向等)。手性GC分析已经用合成样品建立了许多生物碱的绝对立体化学。GC-MS和GC-FTIR，在某些情况下，结合详细的核磁共振分析，甚至用于结构验证的合成，已经描绘了400多个生物碱的结构。用微探针进行的核磁共振分析现已应用于仅10微克的生物碱样品。目前从中南美洲和马达加斯加两栖动物的提取物中鉴定出约100种新的生物碱，其中一些代表新的结构类别，包括N-甲基十氢喹啉和脱氢肼。作为某些两栖类皮肤生物碱的食物来源的螨类、蚂蚁、甲虫和千足虫已被鉴定出来，特别是粉尘类毒素和某些齐丁类的粉尘类，蚂蚁类的其他齐丁类，蝙蝠类的甲虫和螺吡咯烷类的管足类千足虫。此外，还从结构上定义了蚂蚁中的新生物碱。蜘蛛蛾(Eudulophasia)幼虫从植物(Spigelia)中截留兰尼定的作用已被发现。其中一种毒素PTX 251D对蚊子具有对映体选择性接触毒性。生物碱的主要生物靶标似乎既是电压敏感的离子通道，又是配体门控的离子通道，特别是钠、钙和烟碱通道。某些毒素被发现激活伤害性感觉通路，可能是通过与钠通道的相互作用。对烟碱通道生物碱活性的分析仍在继续，包括对胰岛素瘤细胞系中发现的高密度神经节型烟碱通道的研究。在体内(小鼠)，正在研究尼古丁激动剂、拮抗剂和正性调节剂，如加兰他明，以寻找治疗阿尔茨海默病的潜在药物。

Amphibian skin has provided a wide range of biologically active alkaloids, many of which have unique profiles of pharmacological activity and therapeutic potential. These alkaloids include batrachotoxins, which are potent activators of sodium channels, histrionicotoxins, which are noncompetitive blockers of nicotinic receptor-channels, pumiliotoxins/allopumiliotoxins/homopumiliotoxins and related congeners, some of which have myotonic and cardiotonic activity due to effects on sodium channels, epibatidine, an extremely potent and selective nicotinic agonist with potent antinociceptive activity and epiquinamide, a nicotinic agonist selective for certain receptor subtypes. Further alkaloids include decahydroquinolines, pyrrolizidines, indolizidines, quinolizidines, lehmizidines, and a variety of tricyclic alkaloids, including spiropyrrolizidine oximes, gephyrotoxins, pseudophrynamines, cyclopentaquinolizidines, coccinellines and coccinelline analogs. The batrachotoxins were also found to occur in certain toxic birds and dietary beetles. Structure elucidation of organic compounds is now based almost exclusively on spectroscopic analysis, using ultraviolet (UV), infrared (IR), mass (MS), and nuclear magnetic resonance (NMR) spectral techniques. Our natural products program has relied on the development of powerful spectral techniques for the analysis of alkaloids and other compounds present in minute amounts in complex mixtures obtained in extracts from amphibian skin and other sources. The key techniques are gas chromatographic (GC) or high performance liquid chromatographic (HPLC) separation, followed by analysis online of UV, IR and MS data. These techniques, along with development of microchemical reactions including hydrogenation, acylation, butylboronation of cis-diols and reductive N-methylation on GC analysis with formaldehyde, have been responsible for the detailed characterization of over 800 alkaloids, representing some 26 structural classes in frog skin extracts. HPLC-MS allows study of all alkaloids, even those of high molecular weight or polarity that do not GC, but gives only limited structural insights because of lack of extensive fragmentation with either atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI). GC-MS analysis using electron impact ionization (EIMS) provides rich, diagnostic patterns of fragmentation, while chemical ionization (CIMS) provides molecular weight and, with deuterated ammonia, the number of exchangeable OH and NH groups. Such pioneering spectroscopic research has been extended to developing and applying tandem mass spectrometry in the collision-activated CIMS mode, demonstrating and elucidating fragmentations different from and complementary to conventional EIMS. The analytical potential of vapor-phase GC-FTIR (Fourier transform IR) has allowed extension from traditional uses of IR (identification of functional groups like OH, carbonyl, double and triple bonds, etc.), to providing valuable stereochemical insights (cis- or trans-ring junctions, Bohlmann band analysis to indicate orientation of hydrogens on carbons adjacent to nitrogen, etc.). Chiral GC analysis has established with synthetic samples the absolute stereochemistry of many alkaloids. GC-MS and GC-FTIR, in conjunction in some cases with detailed NMR analysis and even synthesis for structural verification, have delineated structures of over 400 alkaloids. NMR analysis with microprobe has now been applied to alkaloid samples of only 10 ug. Current extracts from amphibians of Central and South America and Madagascar have led to identification of about 100 new alkaloids, some representing new structural classes, including N-methyldecahydroquinolines and dehydroizidines. Mites, ants, beetles and millipedes that are dietary sources of certain classes of amphibian skin alkaloids have been identified, notably mites for pumiliotoxins and certain izidines, ants for other izidines, melyrid beetles for the batrachotoxins and siphonotid millipedes for the spiropyrrolizidines. Further novel alkaloids from ants have been structurally defined. The sequestration of ryanodine from plants (Spigelia) by larvae of the spider moth (Eudulophasia) has been discovered. One of the pumiliotoxins, namely PTX 251D had enantioselective contact toxicity for mosquitoes. The major biological targets for the alkaloids appear to be both voltage-sensitive and ligand-gated ion channels, in particular sodium, calcium and nicotinic channels. Certain pumiliotoxins were found to activate nociceptive sensory pathways, presumably through interaction with sodium channels. Analysis of the activity of alkaloids at nicotinic channels has continued, including studies with high density ganglionic-type nicotinic channels discovered in an insulinoma cell line. In vivo (mice) effects of nicotinic agonists, antagonists, and positive modulators, such as galanthamine, are being studied in search of potential agents for treatment of Alzheimer's disease.