植物中的多胺代谢

1.植物中多胺的合成代谢

植物中，腐胺（Put）合成的第一步限速反应是由精氨酸脱羧酶（arginine decarboxylase, ADC）催化精氨酸，或者是由鸟氨酸脱羧酶（ornithine decarboxylase, ODC）催化鸟氨酸的脱羧反应。关于拟南芥中是否存在鸟氨酸途径，依旧存有争议。不少人在拟南芥中尝试克隆鸟氨酸脱羧酶基因（ODC）但都失败了（Hanfrey et al., 2001），然而，Tasssoni等人（2003）却在拟南芥中检测到了假定鸟氨酸脱羧酶的活性。亚精胺合成酶（SPDS, EC 2.5.1.16）催化了从腐胺到亚精胺的反应，将氨丙基转移到胺受体上形成亚精胺，而后，再由精胺合成酶（SPMS, EC 2.1.22）进一步催化合成了精胺（Alcázar et al., 2010）。氨丙基基团的另一个供体是脱羧S-腺苷甲硫氨酸（decarboxylated S-adenosylmethionine, dcSAM），S-腺苷甲硫氨酸（S-adenosylmethionine, SAM）由SAM脱羧酶催化（SAM decarboxylase， SAMDC; EC 4.1.1.50）得到dcSAM（Slocum et al., 1984）。（见图2）

拟南芥中，多胺合成酶都是由多个基因编码。例如，编码精氨酸脱羧酶ADC的有两个基因，分别是ADC1和 ADC2，编码亚精胺合成酶的有两个，SPDS1和SPDS2，编码精胺合成酶SPMS的也有两个，SPMS和acaulis 5 （ACL5）。SAMDC的编码基因则有四个，SAMDC1 到SAMDC4（见图2）（Takahashi and Kakehi, 2010）。

2.植物中多胺的分解代谢

多胺的分解涉及到一个或者多个二胺氧化酶（diamine oxidases, DAO, EC 1.4.3.6）和（含黄素腺嘌呤二核苷酸）多胺氧化酶［（flavin adenine dinucleotide）FAD-containing polyamine oxidases, PAO; EC 1.5.3.11）］。这些氧化酶将碳氧化后连接到次级或者初级氨基团上（Moschou et al., 2012; Tavladoraki et al., 2012; 2016）。拟南芥含有五个多胺氧化酶基因，分别是AtPAO1 到 AtPAO5，编码位于细胞质或者过氧化物酶体上的多胺氧化酶 （Tavladoraki et al., 2006; Kamada-Nobusada et al., 2008; Takahashi and Kakehi, 2010; D. W. Kim et al., 2014）。包括拟南芥在内的一些物种，精胺由PAO氧化为亚精胺，而后再氧化为腐胺（Put），并产生过氧化氢（H₂O₂）和3-氨基丙醛（3-aminopropanal）（Cona et al., 2006）。多胺的这种合成与氧化分解双向代谢途径在植物比较常见（见图2）（Moschou et al., 2008）。

二胺氧化酶（DAO），是位于质外体或者过氧化物酶体上的含有铜的氨氧化酶（CuAOs）。通常可将二氨腐胺或者尸胺氧化分解为初级氨基基团，产生Δ¹- 吡咯啉（Δ¹-pyrroline）（后进入三羧酸循环），过氧化氢（H₂O₂）和氨离子（NH⁴⁺）（Cohen, 1998; Alcázar et al., 2010）。拟南芥含有10个CuAOs编码基因[AtCuAOα1, AtCuAOα2, AtCuAOα3 (AtCuAO2), AtCuAOβ (ATAO1 or AtAO1), AtCuAOγ1 (AtCuAO1), AtCuAOγ2, AtCuAOδ (AtCuAOδ2), AtCuAOζ (AtCuAO3), AtCuAOε1 (AtCuAOε) and AtCuAOε2 (AtCuAOδ1)]（Planas-Portell et al., 2013; Ghuge et al., 2015; Tavladoraki et al., 2016）。

图2. 植物中多胺代谢 （Alcázar et al., 2010）。ADC, arginine decarboxylase; AIH, agmatine iminohydrolase; CPA, N-carbamoylputrescine amidohydrolase; NATA1: N-Acetyltransferase activity 1; SPDS, spermidine synthase SPMS, spermine synthase; ACL5, acaulis5; TSPMS, thermo -spermine synthase; SAMDC, S-adenosylmethionine decarboxylase; PAO , polyamine oxidases; CuAO, copper-containing amine oxidases.

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 [RAH1]Thermospermine is derived from Spd not Spm.

 [RAH2]In Arabidopsis, PAOs do not catalyze terminal catabolism (shown in figure) but back-conversion reactions.