Anaerobic conditions reductive metabolism

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

The In vitro metabolic reduction of nitro PAHs was first reported in 1967 (124). Since then a number of similar investigations have been conducted however, most of these studies have been performed under only anaerobic conditions. The oxidative metabolism of nitro PAHs has been examined only recently, but as considered in the following sections, both reductive and oxidative metabolic pathways may be important in the metabolic activation of nitro PAHs. 

Nitropyrene. 1-Nitropyrene is the principal nitro PAH found in diesel exhaust (40) and, therefore, has been the subject of intense study. Nachtman and Wei (133) found that under anaerobic conditions, 1-nitropyrene was reduced by hepatic S9, cytosol or microsomes to principally 1-aminopyrene. Only limited reduction occurred in the absence of cofactors, while maximum metabolism was observed in the presence of both FMN and NADPH. Although the microsomal fraction had the greatest specific activity toward 1-nitropyrene metabolism, the cytosol had 30 times the total activity. 

The nature of the radioactivity in the water, soil and fish from the carbon-14 DDT experiment was examined by thin-layer chromatography as shown in Figure 5. The radioactivity in the water was very polar in nature and did not migrate appreciably from the origin. About 78% of the radioactivity in the soil was extracted with methanol. The major metabolite in the extractable fraction was DDD which represented 33% of the total radioactivity. The reductive dechlorination of DDT to DDD is a known pathway under anaerobic conditions and has been shown to be due to microbial metabolism (5). Since carbon-14 DDT was incor-... 

Halogenated aliphatics can be partially or completely degraded under anaerobic conditions through a transformation reaction called reductive de-halogenation. Often a co-metabolic degradation step, reductive dehalogenation... 

TCE is the other major contaminant at the site and is a common groundwater contaminant in aquifers throughout the United States [425]. Since TCE is a suspected carcinogen, the fate and transport of TCE in the environment and its microbial degradation have been extensively studied [25,63, 95,268,426,427]. Reductive dechlorination under anaerobic conditions and aerobic co-metabolic processes are the predominant pathways for TCE transformation. In aerobic co-metabolic processes, oxidation of TCE is catalyzed by the enzymes induced and expressed for the initial oxidation of the growth substrates [25, 63, 268, 426]. Several growth substrates such as methane, propane, butane, phenol, and toluene have been shown to induce oxygenase enzymes which co-metabolize TCE [428]. 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Oxygen is normally readily available to all reasonably well-perfused tissues, but deep inside organs such as the liver, especially the centrilobular area (see chap. 6), there will be a reduction in the oxygen concentration. This is clearly important when both oxidative and reductive pathways are available for a particular substrate. Therefore, as conditions in a particular tissue become more anaerobic, reductive pathways will become more important. This is well illustrated by the metabolism of halo thane where, in the rat, hypoxia will increase reductive metabolism and hepa to toxicity (see chap. 7). Glutathione is an extremely important cofactor, involved in both protection and conjugation. It may be depleted by both of these processes, or under certain circumstances, such as hereditary glucose-6-phosphate deficiency in man, supply may be reduced (see chap. 5). This will clearly influence toxicity, and there are a number of examples discussed in chapter 7 in which it is important. 

The production of the antigens was reduced in rats in which a deutero analogue of halothane was given and the metabolism of which would be reduced by an isotope effect. The antigens could be formed in vitro, but only under aerobic conditions and not under anaerobic conditions, implicating the oxidative pathway rather than the reductive (Fig. 7.77). 

The product of this metabolic sequence, pyruvate, is a metabolite of caitral importance. Its fate depends upon the conditions within a cell and upon the type of cell. When oxygen is plentiful pyruvate is usually converted to acetyl-coenzyme A, but under anaerobic conditions it may be reduced by NADH + H+ to the alcohol lactic acid (Fig. 10-3, step h). This reduction exactly balances the previous oxidation step, that is, the oxidation of glycer-aldehyde 3-phosphate to 3-phospho-glycerate (steps a and b). With a balanced sequence of an oxidation reaction, followed by a reduction reaction, glucose can be converted to lactate in the absence of oxygen, a fermentation process. The lactic acid fermentation occurs not only in certain bacteria but also in our own muscles under conditions of extremely vigorous exercise. It also occurs continuously in some tissues, e.g., the transparent lens and cornea of the eye. 

Methanopterin (20) is a folate analogue that is isolated from an archae-bacteria, Methanosarcina thermophila, and the bacteria produces methane from CO2 under anaerobic conditions [18-24]. In the methane-producing metabolic process (Scheme 2), tetrahydromethanopterin (21) is known to work as a cofactor for the reduction of the Ci unit. Here, 21 accepts a formyl group that originates from CO2 and transforms it into the formyl... 

The fermentation of 1,3PD from glycerol (see Fig. 8.8c) was discovered in the late 19th century [60]. It has since been found that a considerable number of bacteria can use glycerol as a source of carbon and energy under anaerobic conditions and the reaction pathways have been elucidated [61]. Out of every three molecules of glycerol, one is oxidized, phosphorylated into DHAP and subsequently metabolized via the glycolysis pathway and the TCA cycle the other two are converted into 1,3PD, to maintain redox balance (see Fig. 8.9), via dehydration and NADH-driven reduction. 

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