Lignin-degrading enzyme from Phanerochaete chrysosporium: Purification, characterization, and catalytic properties of a unique H(2)O(2)-requiring oxygenase

An extracellular lignin-degrading enzyme from the basidiomycete Phanerochaete chrysosporium Burdsall was purified to homogeneity by ion-exchange chromatography. The 42,000-dalton ligninase contains one protoheme IX per molecule. It catalyzes, nonstereospecifically, several oxidations in the alkyl side chains of lignin-related compounds: C(alpha)-C(beta) cleavage in lignin-related compounds of the type aryl-C(alpha)HOH-C(beta)HR-C(gamma)H(2)OH (R = -aryl or -O-aryl), oxidation of benzyl alcohols to aldehydes or ketones, intradiol cleavage in phenylglycol structures, and hydroxylation of benzylic methylene groups. It also catalyzes oxidative coupling of phenols, perhaps explaining the long-recognized association between phenol oxidation and lignin degradation. All reactions require H(2)O(2). The C(alpha)-C(beta) cleavage and methylene hydroxylation reactions involve substrate oxygenation; the oxygen atom is from O(2) and not H(2)O(2). Thus the enzyme is an oxygenase, unique in its requirement for H(2)O(2).

Extracellular Enzyme Production and Synthetic Lignin Mineralization by Ceriporiopsis subvermispora

The ability of the white rot fungus Ceriporiopsis subvermispora to mineralize C-synthetic lignin was studied under different culture conditions, and the levels of two extracellular enzymes were monitored. The highest mineralization rates (28% after 28 days) were obtained in cultures containing a growth-limiting amount of nitrogen source (1.0 mM ammonium tartrate); under this condition, the levels of manganese peroxidase (MnP) and laccase present in the culture supernatant solutions were very low compared with cultures containing 10 mM of the nitrogen source. In contrast, cultures containing a limiting concentration of the carbon source (0.1% glucose) showed low levels of both enzymes and also very low mineralization rates compared with cultures containing 1% glucose. Cultures containing 11 ppm of Mn(II) showed a higher rate of mineralization than those containing 0.3 or 40 ppm of this cation. Levels of MnP and laccase were higher when 40 ppm of Mn(II) was used. Mineralization rates were slightly higher in cultures flushed daily with oxygen, whereas laccase levels were lower and MnP levels were approximately the same as in cultures maintained under an air atmosphere. The presence of 0.4 mM veratryl alcohol reduced both mineralization rates and MnP levels, without affecting laccase levels. Lignin peroxidase activity was not detected under any condition. Addition of purified lignin peroxidase to the cultures in the presence or absence of veratryl alcohol did not enhance mineralization rates significantly.

Requirement for a growth substrate during lignin decomposition by two wood-rotting fungi

Decomposition of C-labeled lignin to CO(2) by the lignin-decomposing fungi Phanerochaete chrysosporium and Coriolus versicolor required a growth substrate such as cellulose or glucose. Growth with lignin as sole carbon addition to an otherwise complete medium was negligible.

Influence of Molecular Size and Ligninase Pretreatment on Degradation of Lignins by Xanthomonas sp. Strain 99

The purpose of this study was to examine the relationship between the molecular size of lignin in several preparations and extent of degradation (mineralization) by Xanthomonas sp. strain 99. The influence of ligninase pretreatment was also examined. Five synthetic lignins and one C-methylated spruce lignin were used. The extent of mineralization to CO(2) was greatest for the samples containing the most low-molecular-weight material, and the low-molecular-weight portions were preferentially (or perhaps solely) degraded. Pretreatment of the five synthetic lignins with crude ligninase increased their molecular size and decreased their degradability by the xanthomonad. Pretreatment of the methylated spruce lignin with crude ligninase caused both polymerization and depolymerization but resulted in a net decrease in bacterial degradability. Our results suggest that the xanthomonad can degrade lignins only up to a molecular weight of 600 to 1,000.

Degradation of Phenolic Compounds and Ring Cleavage of Catechol by Phanerochaete chrysosporium

POL-88, a mutant of the white-rot fungus Phanerochaete chrysosporium, was selected for diminished phenol-oxidizing enzyme activity. A wide variety of phenolic compounds were degraded by ligninolytic cultures of this mutant. With several o-diphenolic substrates, degradation intermediates were produced that had UV spectra consistent with muconic acids. Extensive spectrophotometric and polarographic assays failed to detect classical ring-cleaving dioxygenases in cell homogenates or in extracts from ligninolytic cultures. Even so, a sensitive carrier-trapping assay showed that intact cultures degraded [U-C]catechol to [C]muconic acid, establishing the presence of a system capable of 1,2-intradiol fission. Significant accumulation of [C]muconic acid into carrier occurred only when evolution of CO(2) from [C]catechol was inhibited by treating cultures with excess nutrient nitrogen (e.g., l-glutamic acid) or with cycloheximide. l-Glutamic acid is known from past work to repress the ligninolytic system in P. chrysosporium and to mimic the effect of cycloheximide. The results here indicate, therefore, that the enzyme system responsible for degrading ring-cleavage products to CO(2) turns over faster than does the system responsible for ring cleavage.

Microbial decomposition of synthetic C-labeled lignins in nature: lignin biodegradation in a variety of natural materials

Lignin biodegradation in a variety of natural materials was examined using specifically labeled synthetic C-lignins. Natural materials included soils, sediments, silage, steer bedding, and rumen contents. Both aerobic and anaerobic incubations were used. No C-labeled lignin biodegradation to labeled gaseous products under anaerobic conditions was observed. Aerobic C-labeled lignin mineralization varied with respect to type of natural material used, site, soil type and horizon, and temperature. The greatest observed degradation occurred in a soil from Yellowstone National Park and amounted to over 42% conversion of total radioactivity to CO(2) during 78 days of incubation. Amounts of C-labeled lignin mineralization in Wisconsin soils and sediments were significantly correlated with organic carbon, organic nitrogen, nitrate nitrogen, exchangeable calcium, and exchangeable potassium.

Relationship Between Lignin Degradation and Production of Reduced Oxygen Species by Phanerochaete chrysosporium

The relationship between the production of reduced oxygen species, hydrogen peroxide (H(2)O(2)), superoxide (O(2)), and hydroxyl radical (.OH), and the oxidation of synthetic lignin to CO(2) was studied in whole cultures of the white-rot fungus Phanerochaete chrysosporium Burds. The kinetics of the synthesis of H(2)O(2) coincided with the appearance of the ligninolytic system; also, H(2)O(2) production was markedly enhanced by growth under 100% O(2), mimicking the increase in ligninolytic activity characteristic of cultures grown under elevated oxygen tension. Lignin degradation by whole cultures was inhibited by a specific H(2)O(2) scavenger, catalase, implying a role for H(2)O(2) in the degradative process. Superoxide dismutase also inhibited lignin degradation, suggesting that O(2) is also involved in the breakdown of lignin. The production of .OH was assayed in whole cultures by a benzoate decarboxylation assay. Neither the kinetics of .OH synthesis nor the final activity of its producing system obtained under 100% O(2) correlated with that of the lignin-degrading system. However, lignin degradation was inhibited by compounds which react with .OH. It is concluded that H(2)O(2), and perhaps O(2), are involved in lignin degradation; because these species are relatively unreactive per se, their role must be indirect. Conclusions about a role for .OH in ligninolysis could not be reached.

Biosynthetic Pathway for Veratryl Alcohol in the Ligninolytic Fungus Phanerochaete chrysosporium

Veratryl alcohol (VA) is a secondary metabolite of white-rot fungi that produce the ligninolytic enzyme lignin peroxidase. VA stabilizes lignin peroxidase, promotes the ability of this enzyme to oxidize a variety of physiological substrates, and is accordingly thought to play a significant role in fungal ligninolysis. Pulse-labeling and isotope-trapping experiments have now clarified the pathway for VA biosynthesis in the white-rot basidiomycete Phanerochaete chrysosporium. The pulse-labeling data, obtained with C-labeled phenylalanine, cinnamic acid, benzoic acid, and benzaldehyde, showed that radiocarbon labeling followed a reproducible sequence: it peaked first in cinnamate, then in benzoate and benzaldehyde, and finally in VA. Phenylalanine, cinnamate, benzoate, and benzaldehyde were all efficient precursors of VA in vivo. The isotope-trapping experiments showed that exogenous, unlabeled benzoate and benzaldehyde were effective traps of phenylalanine-derived C. These results support a pathway in which VA biosynthesis proceeds as follows: phenylalanine --> cinnamate --> benzoate and/or benzaldehyde --> VA.

Modes of action on cotton and bacterial cellulose of a homologous endoglucanase-exoglucanase pair from Trichoderma reesei

The endoglucanase I (EGI) and the cellobiohydrolase I (CBHI) of the filamentous fungus Trichoderma reesei form a homologous pair of cellulolytic enzymes which nevertheless have different modes of action. We show here that the action of CBHI on bacterial microcrystalline cellulose results in efficient solubilisation but only a slow decrease in its degree of polymerisation. In contrast, the action of EGI results in a rapid decrease of the degree of polymerisation but less efficient overall solubilisation of the substrate. CBHI alone was practically inactive toward cotton which has a high initial degree of polymerisation and a complex morphology. EGI rapidly reduced the degree of polymerisation of cotton, and slowly solubilised part of it. Working synergistically, EGI and CBHI solubilised cotton more rapidly and to a greater extent than EGI alone. Our data are consistent with the exoglucanase nature of CBHI and also provide some evidence supporting its processive mode of action.

The Cellulases Endoglucanase I and Cellobiohydrolase II of Trichoderma reesei Act Synergistically To Solubilize Native Cotton Cellulose but Not To Decrease Its Molecular Size

Degradation of cotton cellulose by Trichoderma reesei endoglucanase I (EGI) and cellobiohydrolase II (CBHII) was investigated by analyzing the insoluble cellulose fragments remaining after enzymatic hydrolysis. Changes in the molecular-size distribution of cellulose after attack by EGI, alone and in combination with CBHII, were determined by size exclusion chromatography of the tricarbanilate derivatives. Cotton cellulose incubated with EGI exhibited a single major peak, which with time shifted to progressively lower degrees of polymerization (DP; number of glucosyl residues per cellulose chain). In the later stages of degradation (8 days), this peak was eventually centered over a DP of 200 to 300 and was accompanied by a second peak (DP, (apprx=)15); a final weight loss of 34% was observed. Although CBHII solubilized approximately 40% of bacterial microcrystalline cellulose, the cellobiohydrolase did not depolymerize or significantly hydrolyze native cotton cellulose. Furthermore, molecular-size distributions of cellulose incubated with EGI together with CBHII did not differ from those attacked solely by EGI. However, a synergistic effect was observed in the reducing-sugar production by the cellulase mixture. From these results we conclude that EGI of T. reesei degrades cotton cellulose by selectively cleaving through the microfibrils at the amorphous sites, whereas CBHII releases soluble sugars from the EGI-degraded cotton cellulose and from the more crystalline bacterial microcrystalline cellulose.

Laccase component of the Ceriporiopsis subvermispora lignin-degrading system

Laccase activity in the lignin-degrading fungus Ceriporiopsis subvermispora was associated with several proteins in the broth of cultures grown in a defined medium. Activity was not increased significantly by adding 2,5-xylidine or supplemental copper to the medium. Higher activity, associated with two major isoenzymes, developed in cultures grown on a wheat bran medium. These two isoenzymes were purified to homogeneity. L1 and L2 had isoelectric points of 3.4 and 4.8, molecular masses of 71 and 68 kDa, and approximate carbohydrate contents of 15 and 10%, respectively. Data indicated 4 copper atoms per mol. L1 and L2 had overlapping pH optima in the range of 3 to 5, depending on the substrate, and exhibited half-lives of 120 and 50 min at 60 degrees C. They were strongly inhibited by sodium azide and thioglycolic acid but not by hydroxylamine or EDTA. The isoenzymes oxidized 1,2,4,5-tetramethoxybenzene but not other methoxybenzene congeners. A variety of usual laccase substrates, including lignin-related phenols and ABTS [2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid)], were also oxidized. Kinetic parameters were similar to those of the laccases of Coriolus versicolor. The N-terminal amino acid sequence (20 residues for L1) showed significant homology to those of laccases of other white rot basidiomycetes but not to those of the laccases of Agaricus bisporus or Neurospora crassa.

Changes in the molecular-size distribution of insoluble celluloses by the action of recombinant Cellulomonas fimi cellulases

Specific patterns of attacks of cotton, bacterial cellulose and bacterial microcrystalline cellulose (BMCC) by recombinant cellulases of Cellulomonas fimi were investigated. Molecular-size distributions of the celluloses were determined by high-performance size-exclusion chromatography. Chromatography of cotton and bacterial celluloses revealed single major peaks centered over progressively lower molecular-mass positions during attack by endoglucanase CenA. In advanced stages, a second peak appeared at very low average size (approx. 11 glucosyl units); ultimate weight losses were approximately 30%. The isolated catalytic domain of CenA, p30, gave results very similar to those with complete CenA. CenA did not effectively depolymerize or solubilize BMCC significantly. Molecular-size distributions of cotton and bacterial cellulose incubated with endoglucanases CenB or CenD exhibited one major peak regardless of incubation time; low-molecular-mass fragments did not accumulate. Weight losses were 40 and 35% respectively. The single peak shifted to lower-molecular-mass positions as incubation continued, but high-molecular-mass material persisted. CenB and CenD readily attacked and solubilized BMCC (approx. 70%). We conclude that CenA attacks cellulose by preferentially cleaving completely through the cellulose microfibrils at the amorphous sites, and much more slowly by degrading the crystalline surfaces. Conversely, CenB and CenD cleave the amorphous regions much less efficiently while vigorously degrading the surfaces of the crystalline regions of the microfibrils.

Three Native Cellulose-Depolymerizing Endoglucanases from Solid-Substrate Cultures of the Brown Rot Fungus Meruliporia (Serpula) incrassata

Three extracellular cellulose-depolymerizing enzymes from cotton undergoing decay by the brown rot fungus Meruliporia (Serpula) incrassata were isolated by anion-exchange and hydrophobic interaction chromatographies. Depolymerization was detected by analyzing the changes in the molecular size distribution of cotton cellulose by high-performance size-exclusion chromatography. The average degree of polymerization (DP; number of glucosyl residues per cellulose chain) was calculated from the size-exclusion chromatography data. The very acidic purified endoglucanases, Cel 25, Cel 49, and Cel 57, were glycosylated and had molecular weights of 25,200, 48,500, and 57,100, respectively. Two, Cel 25 and Cel 49, depolymerized cotton cellulose and were also very active on carboxymethyl cellulose (CMC). Cel 57, by contrast, significantly depolymerized cotton cellulose but did not release reducing sugars from CMC and only very slightly reduced the viscosity of CMC solutions. Molecular size distributions of cotton cellulose attacked by the three endoglucanases revealed single major peaks that shifted to lower DP positions. A second smaller peak (DP, 10 to 20) was also observed in the size-exclusion chromatograms of cotton attacked by Cel 49 and Cel 57. Under the reaction conditions used, Cel 25, the most active of the cellulases, reduced the weight average DP from 3,438 to 315, solubilizing approximately 20% of the cellulose. The weight average DP values of cotton attacked under the same conditions by Cel 49 and Cel 57 were 814 and 534; weight losses were 9 and 11% respectively.

Changes in Molecular Size Distribution of Cellulose during Attack by White Rot and Brown Rot Fungi

The kinetics of cotton cellulose depolymerization by the brown rot fungus Postia placenta and the white rot fungus Phanerochaete chrysosporium were investigated with solid-state cultures. The degree of polymerization (DP; the average number of glucosyl residues per cellulose molecule) of cellulose removed from soil-block cultures during degradation by P. placenta was first determined viscosimetrically. Changes in molecular size distribution of cellulose attacked by either fungus were then determined by size exclusion chromatography as the tricarbanilate derivative. The first study with P. placenta revealed two phases of depolymerization: a rapid decrease to a DP of approximately 800 and then a slower decrease to a DP of approximately 250. Almost all depolymerization occurred before weight loss. Determination of the molecular size distribution of cellulose during attack by the brown rot fungus revealed single major peaks centered over progressively lower DPs. Cellulose attacked by P. chrysosporium was continuously consumed and showed a different pattern of change in molecular size distribution than cellulose attacked by P. placenta. At first, a broad peak which shifted at a slightly lower average DP appeared, but as attack progressed the peak narrowed and the average DP increased slightly. From these results, it is apparent that the mechanism of cellulose degradation differs fundamentally between brown and white rot fungi, as represented by the species studied here. We conclude that the brown rot fungus cleaved completely through the amorphous regions of the cellulose microfibrils, whereas the white rot fungus attacked the surfaces of the microfibrils, resulting in a progressive erosion.

Limited bacterial mineralization of fungal degradation intermediates from synthetic lignin

The ability of selected bacterial strains and consortia to mineralize degradation intermediates produced by Phanerochaete chrysosporium from 14C-labeled synthetic lignins was studied. Three different molecular weight fractions of the intermediates were subjected to the action of the bacteria, which had been grown on a lignin-related dimeric compound. Two consortia isolated from wood being decayed naturally by a Ganoderma species of white rot fungus (the palo podrido system) mineralized 10 to 11% of the fraction with a molecular weight of approximately 500 but less than 4% of the higher- and lower-molecular-weight fractions. The consortia mineralized 5 to 9% of the original lignins. The ability of two pseudomonads isolated earlier from lignin-rich environments to mineralize the original lignins or fungus degradation products was much lower.

Proton NMR investigation into the basis for the relatively high redox potential of lignin peroxidase

Lignin peroxidase shares several structural features with the well-studied horseradish peroxidase and cytochrome c peroxidase but carries a higher redox potential. Here the heme domain of lignin peroxidase and the lignin peroxidase cyanide adduct was examined by 1HNMR spectroscopy, including nuclear Overhauser effect and two-dimensional measurements, and the findings were compared with those for horseradish peroxidase and cytochrome c peroxidase. Structural information was obtained on the orientation of the heme vinyl and propionate groups and the proximal and distal histidines. The shifts of the epsilon1 proton of the proximal histidine were found to be empirically related to the Fe3+/Fe2+ redox potentials.

Oxidation of methoxybenzenes by manganese peroxidase and by Mn3+

Manganese peroxidase, produced by some white-rot fungi during lignin degradation, catalyzes the oxidation of Mn2+ to Mn3+. Whereas Mn3+ is known to oxidize phenolic compounds, its role in lignin degradation is not clear. We have used a series of methoxybenzenes with E1/2 values of 1.76-0.81 V (vs saturated calomel electrode) to investigate the oxidizing ability of Mn3+ chelates generated chemically and enzymatically. Although lignin peroxidase has been shown to oxidize high potential congeners, our results show that manganese peroxidase, or physiological concentrations of Mn3+, oxidize only the lower potential congeners. In addition, Mn3+ increased the rate of decay of the cation radical of 1,2,4,5-tetramethoxybenzene. The kinetics of decay continued to be first order, so Mn3+ does not oxidize the cation radical itself, but probably oxidizes a neutral dienyl radical derived from the cation radical. This indicates a possible role for Mn3+ in lignin degradation, as neutral dienyl radicals are proposed to be products of lignin peroxidase action.

Identification of a specific manganese peroxidase among ligninolytic enzymes secreted by Phanerochaete chrysosporium during wood decay

The specific enzymes associated with lignin degradation in solid lignocellulosic substrates have not been identified. Therefore, we examined extracts of cultures of Phanerochaete chrysosporium that were degrading a mechanical pulp of aspen wood. Western blot (immunoblot) analyses of the partially purified protein revealed lignin peroxidase, manganese-dependent peroxidase (MnP), and glyoxal oxidase. The dominant peroxidase, an isoenzyme of MnP (pI 4.9), was isolated, and its N-terminal amino acid sequence and amino acid composition were determined. The results reveal both similarities to and differences from the deduced amino acid sequences from cDNA clones of dominant MnP isoenzymes from liquid cultures. Our results suggest, therefore, that the ligninolytic-enzyme-encoding genes that are expressed during solid substrate degradation differ from those expressed in liquid culture or are allelic variants of their liquid culture counterparts. In addition to lignin peroxidase, MnP, and glyoxal oxidase, xylanase and protease activities were present in the extracts of the degrading pulp.

Lignin peroxidase oxidation of Mn2+ in the presence of veratryl alcohol, malonic or oxalic acid, and oxygen

Veratryl alcohol (3,4-dimethoxybenzyl alcohol) appears to have multiple roles in lignin degradation by Phanerochaete chrysosporium. It is synthesized de novo by the fungus. It apparently induces expression of lignin peroxidase (LiP), and it protects LiP from inactivation by H2O2. In addition, veratryl alcohol has been shown to potentiate LiP oxidation of compounds that are not good LiP substrates. We have now observed the formation of Mn3+ in reaction mixtures containing LiP, Mn2+, veratryl alcohol, malonate buffer, H2O2, and O2. No Mn3+ was formed if veratryl alcohol or H2O2 was omitted. Mn3+ formation also showed an absolute requirement for oxygen, and oxygen consumption was observed in the reactions. This suggests involvement of active oxygen species. In experiments using oxalate (a metabolite of P. chrysosporium) instead of malonate, similar results were obtained. However, in this case, we detected (by ESR spin-trapping) the production of carbon dioxide anion radical (CO2.-) and perhydroxyl radical (.OOH) in reaction mixtures containing LiP, oxalate, veratryl alcohol, H2O2, and O2. Our data indicate the formation of oxalate radical, which decays to CO2 and CO2.-. The latter reacts with O2 to form O2.-, which then oxidizes Mn2+ to Mn3+. No radicals were detected in the absence of veratryl alcohol. These results indicate that LiP can indirectly oxidize Mn2+ and that veratryl alcohol is probably a radical mediator in this system.

Sensitivity to and Degradation of Pentachlorophenol by Phanerochaete spp

This research measured mycelial extension rates of selected strains of Phanerochaete chrysorhiza, Phanerochaete laevis, Phanerochaete sanguinea, Phanerochaete filamentosa, Phanerochaete sordida, Inonotus circinatus , and Phanerochaete chrysosporium and the ability of these organisms to tolerate and degrade the wood preservative pentachlorophenol (PCP) in an aqueous medium and in soil. Most of the tested species had mycelial extension rates in the range of ≤0.5 to 1.5 cm day −1 , but there were large interspecific differences. A notable exception, P. sordida , grew very rapidly, with an average mycelial extension rate of 2.68 cm day −1 at 28°C. Rank of species by growth rate was as follows: P. chrysosporium > P. sordida > P. laevis > P. chrysorhiza = P. sanguinea > I. circinatus = P. filamentosa. There were also significant intraspecific differences in mycelial extension rates. For example, mycelial extension rates among strains of P. sordida ranged from 1.78 to 4.81 cm day −1 . Phanerochaete spp. were very sensitive to PCP. Growth of several species was prevented by the presence of 5 ppm (5 μg/g) PCP. However, P. chrysosporium and P. sordida grew at 25 ppm PCP, albeit at greatly decreased mycelial extension rates. In an aqueous medium, mineralization of PCP by P. sordida 13 (ca. 12% after 30 days) was significantly greater than that by all other tested P. sordida strains and P. chrysosporium. After 64 days, the level of PCP had decreased by 96 and 82% in soil inoculated with P. chrysosporium and P. sordida , respectively. Depletion of PCP by these fungi occurred in a two-stage process. The first stage was characterized by a rapid depletion of PCP that coincided with an accumulation of pentachloroanisole (PCA). At the end of the first stage, ca. 64 and 71% of the PCP was converted to PCA in P. chrysosporium and P. sordida cultures, respectively. In the second stage, levels of PCP and PCA were reduced by 9.6 and 18%, respectively, in soil inoculated with P. chrysosporium and by 3 and 23%, respectively, in soil inoculated with P. sordida. PCA was mineralized by both P. chrysosporium and P. sordida in an aqueous medium.

Comparison of lignin peroxidase, horseradish peroxidase and laccase in the oxidation of methoxybenzenes

Lignin peroxidase oxidizes non-phenolic substrates by one electron to give aryl-cation-radical intermediates, which react further to give a variety of products. The present study investigated the possibility that other peroxidative and oxidative enzymes known to catalyse one-electron oxidations may also oxidize non-phenolics to cation-radical intermediates and that this ability is related to the redox potential of the substrate. Lignin peroxidase from the fungus Phanerochaete chrysosporium, horseradish peroxidase (HRP) and laccase from the fungus Trametes versicolor were chosen for investigation with methoxybenzenes as a homologous series of substrates. The twelve methoxybenzene congeners have known half-wave potentials that differ by as much as approximately 1 V. Lignin peroxidase oxidized the ten with the lowest half-wave potentials, whereas HRP oxidized the four lowest and laccase oxidized only 1,2,4,5-tetramethoxybenzene, the lowest. E.s.r. spectroscopy showed that this congener is oxidized to its cation radical by all three enzymes. Oxidation in each case gave the same products: 2,5-dimethoxy-p-benzoquinone and 4,5-dimethoxy-o-benzoquinone, in a 4:1 ratio, plus 2 mol of methanol for each 1 mol of substrate. Using HRP-catalysed oxidation, we showed that the quinone oxygen atoms are derived from water. We conclude that the three enzymes affect their substrates similarly, and that whether an aromatic compound is a substrate depends in large part on its redox potential. Furthermore, oxidized lignin peroxidase is clearly a stronger oxidant than oxidized HRP or laccase. Determination of the enzyme kinetic parameters for the methoxybenzene oxidations demonstrated further differences among the enzymes.

Ligninase-mediated phenoxy radical formation and polymerization unaffected by cellobiose:quinone oxidoreductase

Phanerochete chrysosporium ligninase (+ H2O2) oxidized the lignin substructure-related compound acetosyringone to a phenoxy radical which was identified by ESR spectroscopy. Cellobiose:quinone oxidoreductase (CBQase) + cellobiose, previously suggested to be a phenoxy radical reducing system, was without effect on the radical. Ligninase polymerized guaiacol and it increased the molecular size of a synthetic lignin. These polymerizations, reflecting phenoxy radical coupling reactions, were also unaffected by the CBQase system. We conclude that ligninase catalyzes phenol polymerization via phenoxy radicals, which CBQase does not affect. The CBQase system also did not produce H2O2, and its physiological role remains obscure. Glucose oxidase + glucose did produce H2O2 as expected, but, like CBQase, it did not reduce the phenoxy radical of acetosyringone. Because intact cultures of P. chrysosporium depolymerize lignins, it is likely that phenol polymerization by ligninase is prevented or reversed in vivo by an as yet undescribed system.

Metabolism of Lignin Model Compounds of the Arylglycerol-β-Aryl Ether Type by Pseudomonas acidovorans D 3

A natural bacterial isolate that we have classified as Pseudomonas acidovorans grows on the lignin model compounds 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (compound 1) and 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (compound 1′), as well as on the corresponding 1-oxo compounds (2 and 2′) as sole sources of carbon and energy. Metabolic intermediates present in cultures growing on compound 1 included compound 2, 2-methoxyphenol (guaiacol [compound 3]), β-hydroxypro-pioveratrone (compound 4), acetoveratrone (compound 5), and veratric acid (compound 6). Also identified were compounds 1′, 2′, β-hydroxypropiovanillone (compound 4′), and acetovanillone (compound 5′), indicating that 4-O demethylation also occurs. The phenolic intermediates were the same as those found in cultures growing on compound 1′. Compounds 2 and 2′ were in part also reduced to compounds 1 and 1′, respectively. Compound 3 was shown to be derived from the 2-methoxyphenoxy moiety. A suggested degradation scheme is as follows: compound 1→2→(3 + 4)→5→6 (and similarly for 1′). In this scheme, the key reaction is cleavage of the ether linkage between C-2 (C β ) of the phenylpropane moiety and the 2-methoxyphenoxy moiety in compounds 2 and 2′ (i.e., β-aryl ether cleavage). On the basis of compounds identified, viz., 3 and 4 (4′), cleavage appears formally to be reductive. Because this is unlikely, the initial cleavage products probably were not detected. The implications of these results for the enzyme(s) responsible are discussed.

Involvement of a new enzyme, glyoxal oxidase, in extracellular H2O2 production by Phanerochaete chrysosporium

The importance of extracellular H2O2 in lignin degradation has become increasingly apparent with the recent discovery of H2O2-requiring ligninases produced by white-rot fungi. Here we describe a new H2O2-producing activity of Phanerochaete chrysosporium that involves extracellular oxidases able to use simple aldehyde, alpha-hydroxycarbonyl, or alpha-dicarbonyl compounds as substrates. The activity is expressed during secondary metabolism, when the ligninases are also expressed. Analytical isoelectric focusing of the extracellular proteins, followed by activity staining, indicated that minor proteins with broad substrate specificities are responsible for the oxidase activity. Two of the oxidase substrates, glyoxal and methylglyoxal, were also identified, as their quinoxaline derivatives, in the culture fluid as secondary metabolites. The significance of these findings is discussed with respect to lignin degradation and other proposed systems for H2O2 production in P. chrysosporium.

Selection and Improvement of Lignin-Degrading Microorganisms: Potential Strategy Based on Lignin Model-Amino Acid Adducts

The purpose of this investigation was to test a potential strategy for the ligninase-dependent selection of lignin-degrading microorganisms. The strategy involves covalently bonding amino acids to lignin model compounds in such a way that ligninase-catalyzed cleavage of the models releases the amino acids for growth nitrogen. Here we describe the synthesis of glycine- N -2-(3,4-dimethoxyphenyl)ethane-2-ol (I) and demonstrate that growth (as measured by mycelial nitrogen content) of the known lignin-degrading basidiomycete Phanerochaete chrysosporium Burds. with compound I as the nitrogen source depends on its production of ligninase. Ligninase is shown to catalyze the oxidative C—C cleavage of compound I, releasing glycine, formaldehyde, and veratraldehyde at a 1:1:1 stoichiometry. P. chrysosporium utilizes compound I as a nitrogen source, but only after the cultures enter secondary metabolism (day 3 of growth), at which time the ligninase and the other components of the ligninolytic system (lignin → CO 2 ) are expressed. Compound I and related adducts have potential not only in the isolation of lignin-degrading microbes but, perhaps of equal importance, in strain improvement.

Oxidation of polycyclic aromatic hydrocarbons and dibenzo[p]-dioxins by Phanerochaete chrysosporium ligninase

The lignin peroxidase (ligninase) of Phanerochaete chrysosporium catalyzes the oxidation of a variety of lignin-related compounds. Here we report that this enzyme also catalyzes the oxidation of certain aromatic pollutants and compounds related to them, including polycyclic aromatic hydrocarbons with ionization potentials less than or equal to approximately 7.55 eV. This result demonstrates that the H2O2-oxidized states of lignin peroxidase are more oxidizing than the analogous states of classical peroxidases. Experiments with pyrene as the substrate showed that pyrene-1,6-dione and pyrene-1,8-dione are the major oxidation products (84% of total as determined by high performance liquid chromatography), and gas chromatography/mass spectrometry analysis of ligninase-catalyzed pyrene oxidations done in the presence of H2(18)O showed that the quinone oxygens come from water. We found that whole cultures of P. chrysosporium also transiently oxidize pyrene to these quinones. Experiments with dibenzo[p]dioxin and 2-chlorodibenzo[p]dioxin showed that they are also substrates for ligninase. The immediate product of dibenzo[p]dioxin oxidation is the dibenzo[p]dioxin cation radical, which was observed in enzymatic reactions by its electron spin resonance and visible absorption spectra. The cation radical mechanism of ligninase thus applies not only to lignin, but also to other environmentally significant aromatics.

Role of Veratryl Alcohol in Regulating Ligninase Activity in Phanerochaete chrysosporium

Ligninase activity in Phanerochaete chrysosporium is stimulated by incubating cultures with various substrates for the enzyme, including veratryl (3,4-dimethoxybenzyl) alcohol, which is a secondary metabolite of this fungus. This study was designed to provide insight into the mechanism involved in this stimulation. Ligninase activity increased 2 to 4 h after the addition of exogenous veratryl alcohol to ligninolytic cultures. This increase was prevented by inhibitors of protein synthesis. Analysis of the extracellular proteins by high-performance anion-exchange liquid chromatography revealed increases in the amounts of some, but not all, ligninase species. The normal rapid decrease in ligninase activity in aging cultures was not prevented or retarded by veratryl alcohol, indicating that veratryl alcohol does not increase ligninase activity by protecting extant enzyme. We conclude that veratryl alcohol probably functions via an induction type of mechanism, affecting only certain ligninase species. Results with an isolated lignin indicate that lignin (or its biodegradation products) functions in the same way that veratryl alcohol does.

Substrate free radicals are intermediates in ligninase catalysis

The H2O2-requiring ligninase of the basidiomycete Phanerochaete chrysosporium oxidatively cleaves both lignin and lignin model compounds between C alpha and C beta (C-1 and C-2) of their aliphatic side chains. Previous work has demonstrated a reaction mechanism by which ligninase oxidizes aromatic substrates to their cation radicals, which then undergo side chain cleavage to yield carbon-centered free radicals. These carbon-centered radicals add O2 to give substrate peroxyl radicals that react further to yield the hydroxylated and carbonylated end products usually seen in experiments with ligninase. To investigate this radical mechanism, we have now designed three dimeric lignin models: 1-(3,4-dimethoxyphenyl)-2-phenylethanol (I), 1-(3,4-dimethoxyphenyl)-2-phenylpropanol (II), and 1-(3,4-dimethoxyphenyl)-2-methyl-2-phenylpropanol (III). The following results were obtained when these models were oxidized by ligninase: methyl groups at C beta of the substrate favored C alpha-C beta cleavage versus C alpha oxidation to the ketone. GC/MS and HPLC analysis showed that II gave a radical coupling dimer, 2,3-diphenylbutane, as a major (26% yield) reaction product under anaerobic conditions. The anaerobic oxidation of III yielded 2,3-dimethyl-2,3-diphenylbutane. Spin-trapping experiments with nitrosobenzene showed that model II oxidation produced alpha-methylbenzyl radicals, whereas model III oxidation gave alpha, alpha-dimethylbenzyl radicals. TLC and iodometric determinations showed that III gave cumene hydroperoxide as a major (21% yield) reaction product in air. These findings demonstrate that carbon-centered and peroxyl radicals at C beta are major products of C alpha-C beta cleavage by ligninase.

Ligninase of Phanerochaete chrysosporium. Mechanism of its degradation of the non-phenolic arylglycerol beta-aryl ether substructure of lignin

This study examined the ligninase-catalysed degradation of lignin model compounds representing the arylglycerol beta-aryl ether substructure, which is the dominant one in the lignin polymer. Three dimeric model compounds were used, all methoxylated in the 3- and 4-positions of the arylglycerol ring (ring A) and having various substituents in the beta-ether-linked aromatic ring (ring B), so that competing reactions involving both rings could be compared. Studies of the products formed and the time courses of their formation showed that these model compounds are oxidized by ligninase (+ H2O2 + O2) in both ring A and ring B. The major consequence with all three model compounds is oxidation of ring A, leading primarily to cleavage between C(alpha) and C(beta) (C(alpha) being proximal to ring A), and to a lesser extent to the oxidation of the C(alpha)-hydroxy group to a carbonyl group. Such C(alpha)-oxidation deactivates ring A, leaving only ring B for attack. Studies with C(alpha)-carbonyl model compounds corresponding to the three basic model compounds revealed that oxidation of ring B leads in part to dealkoxylations (i.e. to cleavage of the glycerol beta-aryl ether bond and to demethoxylations), but that these are minor reactions in the model compounds most closely related to lignin. Evidence is also given that another consequence of oxidation of ring B in the C(alpha)-carbonyl model compounds is formation of unstable cyclohexadienone ketals, which can decompose with elimination of the beta-ether-linked aromatic ring. The mechanisms proposed for the observed reactions involve initial formation of aryl cation radicals in either ring A or ring B. The cation radical intermediate from one of the C(alpha)-carbonyl model compounds was identified by e.s.r. spectroscopy. The mechanisms are based on earlier studies showing that ligninase acts by oxidizing appropriately substituted aromatic nuclei to aryl cation radicals [Kersten, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 2609-2612; Hammel, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 8348-8353].

Steady-state and transient-state kinetic studies on the oxidation of 3,4-dimethoxybenzyl alcohol catalyzed by the ligninase of Phanerocheate chrysosporium Burds

Catalysis of the H2O2-dependent oxidation of 3,4-dimethoxybenzyl (veratryl) alcohol by the hemoprotein ligninase isolated from wood-decaying fungus, Phanerochaete chrysosporium Burds, is characterized. The reaction yields veratraldehyde and exhibits a stoichiometry of one H2O2 consumed per aldehyde formed. Ping-pong steady-state kinetics are observed for H2O2 (KM = 29 microM) and veratryl alcohol (KM = 72 microM) at pH 3.5. The magnitude of the turnover number varies from 2 to 3 s-1 at this pH, depending on the preparation of the enzyme. Each preparation of enzyme consists of a mixture of active and inactive enzyme. Extensive steady-state kinetic studies of several different preparations of enzyme, suggest a mechanism in which H2O2 reacts with enzyme to form an intermediate that subsequently reacts with the alcohol to return the enzyme to the resting state. The pH dependence of the overall reaction indicates that an ionization occurs having an apparent pK alpha approximately 3.1. The activity is, thus, nearly zero at pH 5 and increases to a maximum near pH approximately 2. However, the enzyme is unstable at this low pH. Transient-state kinetic studies reveal that, upon reaction of ligninase with H2O2, spectral changes occur in the Soret region, which, by analogy to previous studies of horseradish peroxidase, are consistent with formation of Compounds I and II. The active form of the enzyme appears to react rapidly with H2O2; we observed a positive correlation between the turnover number of the enzyme preparation and the extent of a rapid reaction between H2O2 and ligninase to form Compound I. Free radical cations derived from veratryl alcohol do not appear to be released from the enzyme during catalysis; however, other substrates are known to be converted to cation radicals (Kersten, P., Tien, M., Kalyanaraman, B., and Kirk, T.K. (1985) J. Biol. Chem. 260, 2609-2612). Our results are generally consistent with a classical peroxidase mechanism for the action of ligninase on lignin-like substrates.

Production of Ligninases and Degradation of Lignin in Agitated Submerged Cultures of Phanerochaete chrysosporium

Research on the extracellular hemeprotein ligninases of Phanerochaete chrysosporium has been hampered by the necessity to produce them in stationary culture. This investigation examined the effects of detergents on development of ligninase activity in agitated submerged cultures. Results show that addition of Tween 80, Tween 20, or 3-[(3-colamidopropyl)dimethylammonio]1-propanesulfonate to the cultures permits development of ligninase activity comparable to that routinely obtained in stationary cultures. The detergent-amended cultures express the entire ligninolytic system, assayed as the complete oxidation of [ 14 C]lignin to 14 CO 2 . The detergent effect is evidently not merely in facilitating release of extant enzyme. Development of ligninolytic activity in the agitated cultures, as in stationary cultures, is idiophasic. Ion-exchange fast protein-liquid chromatography indicated that the heme protein profiles in agitated and stationary cultures are very similar. These findings should make it possible to scale up production of ligninolytic enzymes in stirred tank fermentors.

Mechanism of oxidative C alpha-C beta cleavage of a lignin model dimer by Phanerochaete chrysosporium ligninase. Stoichiometry and involvement of free radicals

The hemoprotein ligninase of Phanerochaete chrysosporium Burds. catalyzes the oxidative cleavage of lignin model dimers between C alpha and C beta of their propyl side chains. The model dimers hitherto used give multiple products and complex stoichiometries upon enzymatic oxidation. Here we present experiments with a new model dimer, 1-(3,4-dimethoxyphenyl)-2-phenylethanediol (dimethoxyhydrobenzoin, DMHB) which is quantitatively cleaved by ligninase in air to give benzaldehyde and veratraldehyde according to the stoichiometry: 2DMHB + O2----2PhCHO + 2Ph(OMe)2CHO. Catalytic amounts of H2O2 are required for this aerobic reaction. Under anaerobic conditions, ligninase uses H2O2 as the oxidant for cleavage: DMHB + H2O2----PhCHO + Ph(OMe)2CHO. Electron spin resonance experiments done in the presence of spin traps, 2-methyl-2-nitrosopropane or 5,5-dimethyl-1-pyrroline-N-oxide, show that C alpha-C beta cleavage yields alpha-hydroxybenzyl radicals as intermediate products. Under anaerobic conditions, these radicals react further to give the final aldehyde products. In air, O2 adds to the carbon-centered radicals, probably giving alpha-hydroxybenzylperoxyl radicals which fragment to yield superoxide, benzaldehyde, and veratraldehyde. These results lead us to propose a mechanism for C alpha-C beta cleavage in which attack by ligninase and H2O2 on the methoxylated ring of DMHB yields a cation radical, which then cleaves to give either benzaldehyde and an alpha-hydroxy(dimethoxybenzyl) radical or veratraldehyde and an alpha-hydroxybenzyl radical (cf. Kersten, P. J., Tien, M., Kalyanaraman, B., and Kirk, T.K. (1985) J. Biol. Chem. 260, 2609-2612; Snook, M. E., and Hamilton, G. A. (1974) J. Am. Chem. Soc. 96, 860-869). Similar mechanisms probably apply to the enzymatic C alpha-C beta cleavage of natural lignin.

The ligninase of Phanerochaete chrysosporium generates cation radicals from methoxybenzenes

The hemoprotein ligninase of Phanerochaete chrysosporium catalyzes, in the presence of H2O2, a variety of seemingly different oxidations in lignin and lignin model compounds. Here we show that the enzyme also catalyzes the oxidation of various methoxybenzenes. ESR spectroscopy shows that the compounds are oxidized to aryl cation radicals. These decompose, evidently by H2O addition. Thus, 1,4-dimethoxybenzene is converted to p-benzoquinone and methanol. We propose a unified mechanism, based on formation of aryl cation radicals, to explain the various reactions catalyzed by the ligninase.

Factors Involved in the Regulation of a Ligninase Activity in Phanerochaete chrysosporium

The regulation of an H 2 O 2 -dependent ligninolytic activity was examined in the wood decay fungus Phanerochaete chrysosporium. The ligninase appears in cultures upon limitation for nitrogen or carbohydrate and is suppressed by excess nutrients, by cycloheximide, or by culture agitation. Activity is increased by idiophasic exposure of cultures to 100% O 2 . Elevated levels of ligninase and, in some cases, of extracellular H 2 O 2 production are detected after brief incubation of cultures with lignins or lignin substructure models, with the secondary metabolite veratryl alcohol, or with other related compounds. It is concluded that lignin degradation (lignin → CO 2 ) by this organism is regulated in part at the level of the ligninase, which is apparently inducible by its substrates or their degradation products.

Biochemistry of the oxidation of lignin by Phangerochate chrysosporium

The objective of this research was to identify the biochemical agents responsible for the oxidative degradation of lignin by the white-rot fungus Phanerochaete chrysosporium. We examined the hypothesis that activated oxygen species are involved, and we also sought the agent in ligninolytic cultures responsible for a specific oxidative degradative reaction in substructure model compounds. Results of studies of the production of activated oxygen species by cultures, of the effect of their removal on ligninolytic activity, and of their action on substructure model compounds support a role for hydrogen peroxide (H(2)O(2)) and possibly superoxide (O(2)(*)(-)) in lignin degradation. Involvement of hydroxyl radical (*OH) or singlet oxygen (1O(2)) is not supported by our data. The actual biochemical agent responsible for one important oxidative C-C bond cleavage reaction in non-phenolic lignin substructure model compounds, and in lignin itself, was found to be an enzyme. The enzyme is extracellular, has a molecular weight of 42,000 daltons, is azide-sensitive, and requires H(2)O(2) for activity.

A continuous biological process to decolorize bleach plant effluents

Although almost every U.S. pulp mill has a biological wastewater treatment system, these systems based on bacteria, are largely ineffective in the removal of color. For this reason, we have attempted to utilize Phanerochaete chrysosporium, a fungus known to degrade lignin, as the primary organism in a novel waste treatment scheme named the MyCoR Process. Color from bleached Kraft mills originates principally from the first extraction stage of the bleach plant. It is this waste stream which is sent to the MyCoR Process reactor, a rotating biological contactor, for decolorization. We have found that under optimal conditions up to 2,000 color units/L/day can be removed from the waste stream. There is also a concomitant removal of COD and BOD. In addition, chlorolignins originating from the bleaching process were found to be dechlorinated; this is of interest to those concerned with the impact of bleach plant effluents on the environment. The process uses conventional wastewater treatment equipment. However, the use of a pure culture of fungus in a secondary metabolic state has not been attempted previously in a waste treatment scheme. Minor equipment modification and close operator attention may therefore be required. A preliminary economic analysis shows that the MyCoR Process, in its present state, would cost about US$30/metric ton of bleached Kraft pulp produced. This cost will decrease as improved or new strains of fungi are developed for the process.

Lignin-Degrading Enzyme from the Hymenomycete Phanerochaete chrysosporium Burds

The extracellular fluid of ligninolytic cultures of the wood-decomposing basidiomycete Phanerochaete chrysosporium Burds. contains an enzyme that degrades lignin substructure model compounds as well as spruce and birch lignins. It has a molecular size of 42,000 daltons and requires hydrogen peroxide for activity.

Further study discounts role for singlet oxygen in fungal degradation of lignin model compounds

This study reexamined our contention that singlet oxygen (1O2) plays a role in the fungal degradation of lignin (BBRC 102(1981)484). Cultures of Phanerochaete chrysosporium and a photochemical 1O2-generating system (riboflavin/light/O2) cleaved a lignin substructure model compound, 1,2-bis(4-methoxyphenyl)propane-1,3-diol (I), by indistinguishable mechanisms. However, the rate of cleavage of I in D2O was the same as in H2O in the photochemical 1O2-generating system, indicating that 1O2 was not involved. Furthermore, products formed from I in a chemical system for generating 1O2 (H2O2 + NaOCl) differed from those produced by cultures or the photochemical system. It is concluded that 1O2 is not responsible for cleavage of I or related compounds in the fungal cultures or in the photochemical system.

Nutritional Regulation of Lignin Degradation by Phanerochaete chrysosporium

Previous studies have shown that a lignin-degrading system appears in cultures of the white rot fungus Phanerochaete chrysosporium in response to nitrogen starvation, apparently as part of secondary metabolism. We examined the influence of limiting carbohydrate, sulfur, or phosphorus and the effect of varying the concentrations of four trace metals, Ca 2+ , and Mg 2+ . Limitation of carbohydrate or sulfur but not limitation of phosphorus triggered ligninolytic activity. When only carbohydrate was limiting, supplementary carbohydrate caused a transient repression of activity. In carbohydrate-limited cultures, ligninolytic activity appeared when the supplied carbohydrate was depleted, and this activity was associated with a decrease in mycelial dry weight. The amount of lignin degraded depended on the amount of carbohydrate provided, which determined the amount of mycelium produced during primary growth. Carbohydrate-limited cultures synthesized only small amounts of the secondary metabolite veratryl alcohol compared with nitrogen-limited cultures. l -Glutamate sharply repressed ligninolytic activity in carbohydrate-starved cultures, but NH 4 + did not. Ligninolytic activity was also triggered in cultures supplied with 37 μM sulfur as the only limiting nutrient. The balance of trace metals, Mg 2+ , and Ca 2+ was important for lignin degradation.

Ligninolytic enzyme system of Phanaerochaete chrysosporium: synthesized in the absence of lignin in response to nitrogen starvation

The relationship between growth, nutrient nitrogen assimilation, and the appearance of ligninolytic activity was examined in stationary batch cultures of the wood-destroying hymenomycete Phanerochaete chrysosporium Burds. grown under conditions optimized for lignin metabolism. A reproducible sequence of events followed inoculation: 0 to 24 h, germination, linear growth, and depletion of nutrient nitrogen; 24 to 48 h, cessation of linear growth and derepression of ammonium permease activity (demonstrating nitrogen starvation); 72 to 96 h, appearance of ligninolytic activity (synthetic 14C-lignin leads to 14CO2). Experiments with cycloheximide demonstrated that appearance of ligninolytic activity occurs irrespective of the presence of lignin; lignin did not induce additional activity. Addition of NH4+ to cultures immediately prior to the time of appearance of the ligninolytic system delayed its appearance, suggesting that the NH4+ led to interference with synthesis of the enzyme system. Addition of NH4+ to ligninolytic cultures resulted in an eventual, temporary decrease in ligninolytic activity. The results suggest that all or essential protein components of the ligninolytic enzyme system are synthesized as part of a series of physiological ("secondary metabolic") events that are initiated by nutrient nitrogen starvation.

Preparation and microbial decomposition of synthetic [14C]ligins

A definitive assay for microbiological and biochemical research on the biodegradation of lignin was developed using radioactive synthetic lignins specifically labeled in the side chains, aromatic rings or in the methoxyl groups. The [14C]lignins were prepared by oxidative polymerization with peroxidase and H2O2 Of specifically labeled coniferyl alcohol (4-hydroxy-3-methyoxycinnamyl alcohol). The synthetic polymers were shown by spectroscopic and chemical methods to contain the same intermonomer linkages found in natural lignins. Incubation of the [14C]lignins with known lignin-degrading fungi and with a forest soil resulted in 14CO2 evolution.

Dissimilation of the lignin model compound veratrylglycerol-beta-(o-methoxyphenyl) ether by Pseudomonas acidovorans: initial transformations

The initial transformadomonas acidovorans are demethylation of the 4-methoxyl group of the 3,4-dimethoxyphenyl portion of the molecule, and a NAD+-linked dehydrogenation of the benzyl alcohol group. Based on these findings and those described before, an overall degradation scheme is postulated.

Oxygenation of 4-alkoxyl groups in alkoxybenzoic acids by Polyporus dichrous

The degradation of several alkyl ethers of vanillic acid, of 3-ethoxy-4-hydroxybenzoic acid, and of syringic acid, by the lignin-decomposing fungus Polyporus dichrous included (i) 4-dealkylation (e.g., 3-ethoxy-4-isopropoxybenzoic acid was in part dealkylated to 3-ethoxy-4-hydroxybenzoic acid), (ii) hydroxylation of the 4-alkoxyl groups (e.g., 3-ethoxy-4-isopropoxybenzoic acid was oxidized in part to 2-[4-carboxy-2-ethoxyphenoxy]-propane-1-ol), and (iii) reduction of carboxyl groups (older cultures) (e.g., 3-ethoxy-4-isopropoxybenzoic acid was reduced to 3-ethoxy-4-isopropoxybenzaldehyde and 3-ethoxy-4-isopropoxybenzyl alcohol). Some ethers (e.g., tri-O-methyl gallic acid and glycerol-beta-[4-carboxy-2-ethoxyphenyl]-ether) were not affected. The dealkylations and hydroxylations indicate that the fungus has a relatively nonspecific mechanism for oxygenating various 4-alkoxyl groups of alkoxybenzoic acids; no evidence for oxygenation of 3-alkoxyl groups was obtained. Hydroxylation products were generally degraded further, probably via dealkylation. The vanillic acid and 3-ethoxy-4-hydroxybenzoic acid formed by dealkylations were readily metabolized. Although the isopropyl ether of syringic acid was hydroxylated to 2-(4-carboxy-2, 6-dimethoxyphenoxy)-propane-1-ol, neither this compound nor the parent isopropyl ether was dealkylated; syringic acid itself was only slowly and incompletely metabolized. The relationship of these results to lignin degradation is discussed.