Oberhuber Michael, Berghold Joachim, Breuker Kathrin, Hortensteiner Stefan, Krautler Bernhard
Institute of Organic Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria.
Proc Natl Acad Sci U S A. 2003 Jun 10;100(12):6910-5. doi: 10.1073/pnas.1232207100. Epub 2003 May 30.
Senescent higher plants degrade their chlorophylls (Chls) to polar colorless tetrapyrrolic Chl catabolites, which accumulate in the vacuoles. In extracts from degreened leaves of the tree Cercidiphyllum japonicum an unpolar catabolite of this type was discovered. This tetrapyrrole was named Cj-NCC-2 and was found to be identical with the product of a stereoselective nonenzymatic isomerization of a "fluorescent" Chl catabolite. This (bio-mimetic) formation of the "nonfluorescent" catabolite Cj-NCC-2 took place readily at ambient temperature and at pH 4.9 in aqueous solution. The indicated nonenzymatic process is able to account for a crucial step during Chl breakdown in senescent higher plants. Once delivered to the acidic vacuoles, the fluorescent Chl catabolites are due to undergo a rapid, stereoselective isomerization to the ubiquitous nonfluorescent catabolites. The degradation of the Chl macrocycle is thus indicated to rely on just two known enzymes, one of which is senescence specific and cuts open the chlorin macroring. The two enzymes supply the fluorescent Chl catabolites, which are "programmed" to isomerize further rapidly in an acidic medium, as shown here. Indeed, only small amounts of the latter are temporarily observable during senescence in higher plants.
衰老的高等植物会将其叶绿素（Chls）降解为极性无色四吡咯叶绿素分解代谢产物，这些产物会在液泡中积累。在日本连香树落叶的提取物中发现了一种这种类型的非极性分解代谢产物。这种四吡咯被命名为Cj - NCC - 2，并且发现它与一种“荧光”叶绿素分解代谢产物的立体选择性非酶异构化产物相同。“非荧光”分解代谢产物Cj - NCC - 2的这种（仿生）形成在环境温度和pH 4.9的水溶液中很容易发生。所指出的非酶过程能够解释衰老高等植物叶绿素分解过程中的一个关键步骤。一旦被输送到酸性液泡中，荧光叶绿素分解代谢产物就会迅速经历立体选择性异构化，转化为普遍存在的非荧光分解代谢产物。因此表明叶绿素大环的降解仅依赖于两种已知的酶，其中一种是衰老特异性的，它会切开二氢卟吩大环。这两种酶产生荧光叶绿素分解代谢产物，如本文所示，这些产物“被设定程序”在酸性介质中进一步快速异构化。实际上，在高等植物衰老过程中只能暂时观察到少量的后者。
