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A metabolic mechanism for oxalic acid biosynthesis in the woodrotting basidiomycete Fomitopsis palustris has been proposed on the basis of biochemical analyses of glucose metabolism. There was a strong correlation between glucose consumption and oxalate production. Oxalic acid was found to accumulate in the culture fluid in about 80% of the theoretical yield or about 5-fold, on the basis of the fungal biomass harvested. The results clearly indicate that glucose was not completely oxidized to CO2 by the tricarboxylic acid (TCA) cycle but converted mainly to oxalate. The determination of the 12 enzymes concerned has revealed the occurrence of the unprecedented metabolic coupling of the TCA and glyoxylate cycles that support oxalate biosynthesis. In this metabolic system, isocitrate lyase (EC 4.1.3.1), together with oxaloacetase (EC 3.7.1.1), was found to play a pivotal role in yielding oxalate from oxaloacetate via the acetate-recycling routes. Moreover, malate dehydrogenase (EC 1.1.1.37), with an extraordinarily high activity among the enzymes tested, was shown to play an important role in generating NADH by oxidation of malate to oxaloacetate. Thus, it is proposed that the wood-rotting basidiomycete acquires biochemical energy by oxidizing glucose to oxalate.

A lignan, lariciresinol, was isolated from Arabidopsis thaliana, the most widely used model plant in plant bioscience sectors, for the first time. In the A. thaliana genome database, there are two genes (At1g32100 and At4g13660) that are annotated as pinoresinol/lariciresinol reductase (PLR). The recombinant AtPLRs showed strict substrate preference toward pinoresinol but only weak or no activity toward lariciresinol, which is in sharp contrast to conventional PLRs of other plants that can reduce both pinoresinol and lariciresinol efficiently to lariciresinol and secoisolariciresinol, respectively. Therefore, we renamed AtPLRs as A. thaliana pinoresinol reductases (AtPrRs).

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