C3 unit addition reaction accompanied by formation of C-C bond in phytotoxin biosynthesis of phytopathogen.

Sorokinianin是由引起小麦和其它草的斑点病或根腐病的病原体的Bipolaris sorokiniana的菌株（OB-25-l）产生的植物毒素。推测该毒素由一个长蠕孢酚型倍半萜和一个C_3部分组成。为了阐明其生物合成来源，我们进行了三个实验：[1，2-^<13>C_2]乙酸钠补料实验; C_3单元前体的候选物与菌株OB-25-1的主要代谢产物prehelminthosporol一起的置换培养实验;以及[2，3-^<13>C_2]琥珀酸补料实验。这些实验的结果表明，sorokinianin合成从prehelminthosporol的反应，包括添加的TCA循环，脱羧和内酯化的成员。为了确定C_3单元的直接前体，我们<18>用过氧化氢酶和H_2_O从富马酸制备L-[羟基-^ O]苹果酸<18>，并进行了L-[羟基-^<18>O]苹果酸的补料实验。实验结果表明，C_3单元的直接前体不是苹果酸/丁二酸，而是富马酸/丁二酸。

Sorokinianin is a phytotoxin produced by a strain (OB-25-l) of Bipolaris sorokiniana, a pathogen causing spot blotch or foot and root rot diseases of wheat and other grasses. This toxin is assumed to be composed of a helminthosporol-type sesquiterpene and a C_3 portion. To clarify the biosynthetic origin we conducted three experiments : a [1,2-^<13>C_2] sodium acetate feeding experiment ; a replacement culture experiment with the candidates for the C_3 unit precursor, together with prehelminthosporol, the major metabolite of the strain OB-25-1 ; and a [2,3-^<13>C_2] succinic acid feeding experiment. The results of these experiments suggested that sorokinianin was synthesized from prehelminthosporol by reactions that include the addition of a member of the TCA cycle, decarboxylation and lactonization. To determine the direct C_3 unit precursor we prepared L-[hydroxy-^<18>O] malic acid from fumaric acid by fumarase and H_2_<18>O and conducted a L-[hydroxy-^<18>O] malic acid feeding experiment. The results of the experiment indicated that the direct C_3 unit precursor was not malic acid/oxalacetic acid but fumaric acid/succinic acid.