Structures And Biological Activity Of Alkaloids

两栖动物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。这些生物碱包括蛙毒素（batrachotoxins），其是钠通道的有效激活剂;组蛋白毒素（histrionicotoxins），其是烟碱受体通道和钾通道的非竞争性阻断剂; pumiliotoxins/allopumiliotoxins和相关的homopumiliotoxins，其由于对钠通道的作用而具有肌强直和心脏强直活性;以及地棘蛙素（epibatidine），其是具有有效抗伤害感受活性的极其有效和选择性的烟碱激动剂。其他生物碱包括十氢喹啉、吡咯烷、吲嗪、喹嗪、前所未有的一类二取代的氮杂双环[5.3.0]癸烷（lehmizidine）和各种三环生物碱，包括螺吡咯烷肟、gephyrotoxins、假菲林胺、环戊喹嗪、coccinelline和coccinelline类似物。在某些有毒鸟类中也存在蛙毒素。有机化合物的结构解析现在几乎完全基于光谱分析，使用紫外（UV）、红外（IR）、质谱（MS）和核磁共振（NMR）光谱技术。我们的天然产品项目依赖于强大的光谱技术的发展，用于分析从两栖动物皮肤和其他来源提取物中获得的复杂混合物中微量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，然后在线分析UV，IR和MS数据。这些技术，沿着微化学反应的发展（氘交换，氢化，酰化，顺式二醇的丁基硼化，用甲醛进行GC分析的还原性N-甲基化，以及其他微反应），已经负责近600种生物碱的详细表征，代表青蛙皮肤提取物中的20多种结构类别。HPLC是最通用的分离工具，允许研究所有生物碱，即使是那些没有GC的高分子量或极性的生物碱，但由于缺乏大气压化学电离（APCI）或电喷雾电离（ESI）的广泛碎片化，因此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、诊断性的碎片模式，而化学电离（CIMS）提供了分子量，并且使用氘代氨提供了可交换的OH和NH基团的数量。这种开创性的光谱研究已扩展到开发和应用串联质谱的碰撞激活CIMS模式，展示和阐明不同于传统的EIMS和互补的碎片。气相GC-FTIR（傅里叶变换红外）的分析潜力允许从红外的传统用途（识别官能团，如OH、羰基、双键和三键等）扩展，获得立体化学的见解（顺式或反式环连接，使用Bohlmann谱带分析信息来确定氢在与氮相邻的碳上的取向等）。 在某些情况下，结合详细的NMR分析，甚至合成结构验证，已经描绘了300多种生物碱的结构。目前从中南美洲和马达加斯加的两栖动物中提取的生物碱已鉴定出近50种新的生物碱，其中许多代表了新的结构类别。蚂蚁、甲虫和千足虫是某些种类两栖动物皮肤生物碱的饮食来源，已被确定。一个独特的pumiliotoxin 7-羟化酶在一个谱系的新热带区毒蛙立体选择性地转换pumiliotoxin一个更有毒的allopumiliotoxin。 生物碱的主要生物靶点似乎是电压敏感性和配体门控离子通道，特别是钠、钙和烟碱通道。

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 and potassium channels, pumiliotoxins/allopumiliotoxins and related homopumiliotoxins, which have myotonic and cardiotonic activity due to effects on sodium channels, and epibatidine, an extremely potent and selective nicotinic agonist with potent antinociceptive activity. Further alkaloids include decahydroquinolines, pyrrolizidines, indolizidines, quinolizidines, an unprecedented class of disubstituted azabicyclo[5.3.0]decanes (lehmizidines), and a variety of tricyclic alkaloids, including spiropyrrolizidine oximes, gephyrotoxins, pseudophrynamines, cyclopentaquinolizidines, coccinellines and coccinelline analogs. The batrachotoxins also occur in certain toxic birds. 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 (deuterium exchange, hydrogenation, acylation, butylboronation of cis-diols, reductive N-methylation on GC analysis with formaldehyde, and other microreactions) have been responsible for the detailed characterization of nearly 600 alkaloids, representing more than 20 structural classes in frog skin extracts. HPLC is the most general separation tool, allowing study of all alkaloids, even those of high molecular weight or polarity that do not GC, but giving 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 (to identify functional groups like OH, carbonyl, double and triple bonds, etc.), to the obtaining of stereochemical insights (cis- or trans-ring junctions, use of Bohlmann band analysis information as to orientation of hydrogens on carbons adjacent to nitrogen, etc.). In conjunction in some cases with detailed NMR analysis and even synthesis for structural verification, structures of over 300 alkaloids have been delineated. Current extracts from amphibians of Central and South America and Madagascar have led to identification of nearly 50 new alkaloids, many representing new structural classes. Ants, beetles and millipedes that are dietary sources of certain classes of amphibian skin alkaloids have been identified. A unique pumiliotoxin 7-hydroxylase in one lineage of neotropical poison frogs stereoselectively converts a pumiliotoxin to a more toxic allopumiliotoxin. 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.