Dräger Birgit
Faculty of Pharmacy, Martin Luther University Halle-Wittenberg, Hoher Weg 8, D-06120 Halle/Saale, Germany.
Phytochemistry. 2006 Feb;67(4):327-37. doi: 10.1016/j.phytochem.2005.12.001. Epub 2006 Jan 19.
Two stereospecific oxidoreductases constitute a branch point in tropane alkaloid metabolism. Products of tropane metabolism are the alkaloids hyoscyamine, scopolamine, cocaine, and polyhydroxylated nortropane alkaloids, the calystegines. Both tropinone reductases reduce the precursor tropinone to yield either tropine or pseudotropine. In Solanaceae, tropine is incorporated into hyoscyamine and scopolamine; pseudotropine is the first specific metabolite on the way to the calystegines. Isolation, cloning and heterologous expression of both tropinone reductases enabled kinetic characterisation, protein crystallisation, and structure elucidation. Stereospecificity of reduction is achieved by binding tropinone in the respective enzyme active centre in opposite orientation. Immunolocalisation of both enzyme proteins in cultured roots revealed a tissue-specific protein accumulation. Metabolite flux through both arms of the tropane alkaloid pathway appears to be regulated by the activity of both enzymes and by their access to the precursor tropinone. Both tropinone reductases are NADPH-dependent short-chain dehydrogenases with amino acid sequence similarity of more than 50% suggesting their descent from a common ancestor. Putative tropinone reductase sequences annotated in plant genomes other that Solanaceae await functional characterisation.
两种立体特异性氧化还原酶构成了托烷生物碱代谢的一个分支点。托烷代谢产物包括生物碱阿托品、东莨菪碱、可卡因以及多羟基去甲托烷生物碱（即卡利斯廷碱）。两种托品酮还原酶都能将前体托品酮还原，生成托品或假托品。在茄科植物中，托品被整合到阿托品和东莨菪碱中；假托品是通往卡利斯廷碱途中的首个特异性代谢产物。两种托品酮还原酶的分离、克隆及异源表达实现了动力学表征、蛋白质结晶及结构解析。还原反应的立体特异性是通过托品酮以相反方向结合在各自酶的活性中心来实现的。两种酶蛋白在培养根中的免疫定位显示出组织特异性的蛋白质积累。托烷生物碱途径两条分支的代谢通量似乎受两种酶的活性及其对前体托品酮的可及性调控。两种托品酮还原酶都是依赖NADPH的短链脱氢酶，氨基酸序列相似度超过50%，这表明它们源自共同的祖先。茄科以外的植物基因组中注释的推定托品酮还原酶序列有待进行功能表征。
