Veratryl alcohol metabolism

Figure 4. Hypothetical scheme for veratryl alcohol metabolism by ligni-nolytic cultures of P. chrysosporium ...

These findings led to the proposition that the veratryl alcohol is degraded via the quinone intermediates (Figure 5) to CO2 through a series of transformations involving lignin peroxidase, perhydroxy radicals and the NADP-dependent aryl alcohol oxidoreductase. Veratraldehyde, the major product of lignin peroxidase catalyzed veratryl alcohol oxidation, is rapidly reduced back to veratryl alcohol it is the further metabolism of the side products of the oxidative process, viz. the quinones and lactones, that drives the overall transformation towards completion (34). 

The previously proposed metabolism of L-phenylalanine to veratryl alcohol (19-22) is now slightly modified, as shown in Figure 4. This pathway... 

It has now been found that the quinones 13, 14 and 15 from the aerobic oxidation of veratryl alcohol by lignin peroxidase were also reduced by fungal mycelium to yield the corresponding hydroquinones. For quinone 14 this reduction had already been reported by Buswell et al. (17) in a study of vanillic acid metabolism. 

Metabolism of Monomeric Lignin Models. Leisola and coworkers (31,37) have reported on the products of the lignin peroxidase catalyzed oxidation of veratryl alcohol. Veratraldehyde was the major product (> 70% yield), together with a number of minor products, the quinones 13 and 14 and the ringopened lactones 16 and 17. In addition, Shimada et al. (38,39) showed that the 6-lactone 18 was also formed. Recently we obtained evidence that the ortho-quinone 15 and the 6-lactone 19 were also products of the lignin peroxidase catalyzed oxidation of veratryl alcohol (Schmidt ef al, Biochemistry, in press). Mechanisms for the formation of those compounds... 

Leisola et al. (15) have studied the metabolism of veratryl alcohol by ligninolytic cultures of P. chrysosporium. From studies with 14C ring labeled compounds, the metabolic scheme depicted in Figure 4 was postulated. 

In this scheme veratryl alcohol was viewed to be metabolized by the combined action of oxidative systems (the lignin peroxidase and possibly other active oxygen species) and reductive conversions (aldehyde and quinone reductions). A possible route via veratric acid was discounted because both veratraldehyde and veratric acid were not substrates for the lignin peroxidase under the condition studied. However, both veratraldehyde and veratric acid were rapidly and quantitatively reduced by ligninolytic cultures of P. chrysosporium (21). 

In addition to catalyzing the oxidation of many compounds, LiP is also able to catalyze reductive reactions in the presence of electron donors such as EDTAor oxalate (Fig. 7) (6, 77). Veratryl alcohol is a free radical mediator in these reactions. The electron donors appear to be oxidized by a LiP generated veratryl alcohol cation radical. The resulting anion radical can catalyze the reduction of good electron acceptors such as cytochrome c, nitroblue tetrazolium, and oxygen. Evolution of CO2 from EDTA or oxalate effectively drives the reductive reactions. Similar reactions have also been observed with the manganese dependent peroxidases in the presence of quinones (20). Early work performed in our laboratory showed that these reductive mechanisms are not involved in TNT reduction. However, these reactions may be involved in other steps in TNT metabolism. 

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