Structures And Biological Activity Of Alkaloids

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巴曲霉毒素，烟碱受体通道和钾通道的非竞争性阻滞剂组氨酸毒素，通过对钠通道的作用而具有强肌和强心活性的短臂毒素/别杆毒素和相关的高杆毒素，以及具有强大的抗伤害性活性的极其有效和选择性的烟碱激动剂表巴替丁。其他生物碱还包括十氢喹啉、吡咯利齐定、吲哚里西定、喹唑烷、一类前所未有的双取代氮杂双环[5.3.0]癸烷(来咪嗪)，以及各种三环生物碱，包括螺吡咯里西定酮、绿藻毒素、伪菲咯烷、环戊喹啉、球孢菌素和球孢菌素类似物。蝙蝠毒素也存在于某些有毒的禽类中。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。随着微化学反应的发展(氢交换、氢化、酰化、顺式二醇的丁基硼化、甲醛还原N-甲基化和其他微反应)的发展，这些技术已经对近600种生物碱进行了详细的表征，代表了蛙皮提取物中的20多个结构类别。高效液相色谱是最通用的分离工具，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，只能给出有限的结构见解。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(识别羟基、羰基、双键和三键等官能团)扩展到立体化学洞察(顺式或跨环连接，使用Bohlmann带分析信息来确定氢在氮相邻碳上的取向等)。 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300多个生物碱的结构。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近50种新的生物碱，其中许多代表新的结构类别。蚂蚁、甲虫和千足虫是某些两栖类皮肤生物碱的饮食来源，已经被识别出来。在新热带毒蛙的一个谱系中，一种独特的细小毒素7-羟基酶可以立体选择性地将细小毒素转化为毒性更强的别孔毒素。 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。

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.