Inomata Masahiro, Hirai Nobuhiro, Yoshida Ryuji, Ohigashi Hajime
Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan.
Phytochemistry. 2004 Oct;65(19):2667-78. doi: 10.1016/j.phytochem.2004.08.025.
The biosynthetic pathway to abscisic acid (ABA) from isopentenyl diphosphate in the fungus, Botrytis cinerea, was investigated. Labeling experiments with (18)O2 and H2(18)O indicated that all oxygen atoms at C-1, -1, -1' and -4' of ABA were derived from molecular oxygen, and not from water. This finding was inconsistent not only with the known carotenoid pathway via oxidative cleavage of carotenoids, but also with the classical direct pathway via cyclization of farnesyl diphosphate. The fungus produced new C15-compounds, 2E,4E-alpha-ionylideneethane and 2Z,4E-alpha-ionylideneethane, along with 2E,4E,6E-allofarnesene and 2Z,4E,6E-allofarnesene, but did not apparently produce carotenoids except for a trace of phytoene. The C15-compounds labeled with 13C were converted to ABA by the fungus, and the incorporation ratio of 2Z,4E-alpha-ionylideneethane was higher than that of 2E,4E-alpha-ionylideneethane. From these results, it was concluded that farnesyl diphosphate was reduced at C-1, desaturated at C-4, and isomerized at C-2 to form 2Z,4E,6E-allofarnesene before being cyclized to 2Z,4E-alpha-ionylideneethane; the ionylideneethane was then oxidized to ABA with molecular oxygen. This direct pathway via ionylideneethane means that the biosynthetic pathway to fungal ABA, not only before but also after isopentenyl diphosphate, differs from that to ABA in plants, since plant ABA is biosynthesized using the non-mevalonate and carotenoid pathways.
对灰葡萄孢菌中从异戊烯基二磷酸合成脱落酸（ABA）的生物合成途径进行了研究。用（18）O2和H2（18）O进行的标记实验表明，ABA的C-1、-1、-1'和-4'位的所有氧原子均来源于分子氧，而非水。这一发现不仅与已知的通过类胡萝卜素氧化裂解的类胡萝卜素途径不一致，也与通过法呢基二磷酸环化的经典直接途径不一致。该真菌产生了新的C15化合物，2E,4E-α-亚异丁烯基乙烷和2Z,4E-α-亚异丁烯基乙烷，以及2E,4E,6E-金合欢烯和2Z,4E,6E-金合欢烯，但除了微量的八氢番茄红素外，显然没有产生类胡萝卜素。用13C标记的C15化合物被该真菌转化为ABA，且2Z,4E-α-亚异丁烯基乙烷的掺入率高于2E,4E-α-亚异丁烯基乙烷。从这些结果可以得出结论，法呢基二磷酸在C-1位被还原，在C-4位去饱和，并在C-2位异构化形成2Z,4E,6E-金合欢烯，然后环化形成2Z,4E-α-亚异丁烯基乙烷；随后亚异丁烯基乙烷被分子氧氧化为ABA。这条通过亚异丁烯基乙烷的直接途径意味着，真菌ABA的生物合成途径，不仅在异戊烯基二磷酸之前，而且在其之后，都与植物中ABA的生物合成途径不同，因为植物ABA是通过非甲羟戊酸途径和类胡萝卜素途径生物合成的。
