Biosynthesis of Nitro Compounds. III. The Enzymatic Reduction of β-Nitroacrylic Acid to β-Nitropropionic Acid*

Assessment of Soil Contamination by Gas Cloud Generated from Chemical Fire Using Metabolic Profiling and Associated Bacterial Communities

Chemical accidents have frequently occurred in South Korea as a result of the huge amount of chemicals being used in various industries. Even though fire accidents accounted for 71.9% of chemical accidents during 2008–2018 in South Korea, most ecological research and investigation has focused on leakage accidents since most fire or explosion gases are diffused out and disappear into the atmosphere. In this study, the possibility of soil contamination by toluene combustion is proposed. A fire simulation batch test was performed and identified the combustion by-products such as methylbenzene, ethylbenzene, ethynylbenzene, benzaldehyde, 1-phenyl-1-propyne, naphthalene, 2-methylindene using gas chromatography coupled with mass spectrometry (GC–MS). Naphthyl-2-methyl-succinic acid, a metabolic intermediate of naphthalene metabolism derived from the combustion product of toluene, was also discovered in field soil and the secondary metabolites such as streptomycin 6-phosphate, 3-Nitroacrylate, oxaloacetate using LC–MS. Moreover, Streptomyces scabiei, participating in naphthalene metabolism, was also discovered in filed soil (contaminated soil) using 16s rRNA sequencing. As a result, bacterial stress responses in field soil (contaminated soil) affected by gas cloud were identified by discovering metabolites relating to bacterial self-defense action such as fatty biosynthesis. This study draws a conclusion that soil can be polluted enough to affect bacteria by gas cloud and soil bacteria and can encounter stress for a long term even though toluene and its combustion products had already decomposed in soil.

The biochemistry of the metabolic poison propionate 3-nitronate and its conjugate acid, 3-nitropropionate

3-Nitropropionate (3-NPA) is a nitro aliphatic compound found in numerous plants and fungi. The nitro compound exists in equilibrium with its conjugate base, propionate 3-nitronate (P3N) and has a pKa approaching the physiological range of 9.1. Since 1920, more than 30 species of plant and fungi have been identified as producing 3-NPA as a means of defense from herbivores. Glycoside products containing moieties of 3-NPA found in parts of the plants most accessible to herbivores can be easily hydrolyzed to free 3-NPA by bacterial enzymes in the gut of animals. In addition to providing a defense mechanism, the nitro compound is an intermediate in the nitrification process of leguminous plants. The synthesis of 3-NPA in these plants and fungi is poorly understood. P3N, which readily forms from 3-NPA at physiological pH, is a potent inhibitor of the key enzyme succinate dehydrogenase in the Krebs cycle and electron transport chain. Inhibition of succinate dehydrogenase in humans and livestock causes neurotoxicity and in some cases death. Several enzymes catalyze the oxidation of 3-NPA or P3N; all contain a noncovalently bound flavin cofactor and are found in the organisms that produce 3-NPA. With k(cat)/K(m) values of >10(6) M(-1) s(-1), nitronate monooxygenases can quickly and efficiently oxidize P3N to malonic semialdehyde as a means of protecting the organism from killing itself. Although it was discovered almost a century ago, the biochemistry and physiological role of 3-NPA/P3N are just emerging.

Purification and some properties of a hitherto-unknown enzyme reducing the carbon-carbon double bond of alpha, beta-unsaturated carboxylate anions

2-Enoate-reductase, a previously unknown soluble enzyme is present in Clostridium kluyveri and another Clostridium species growing on (E)-2-butenoate. From the latter the reductase was purified 88-fold with an overall yield up to 74%. The enzyme was pure as judged by polyacrylamide gel electrophoresis with and without sodium dodecyl sulphate as well as by isoelectric focusing. The purification of the enzyme was performed in the presence of (E)-2-methyl-2-butenoate as substrate to keep the enzyme in the oxidized state and under anaerobic conditions. The purification procedure included an ammonium sulphate precipitation, chromatography on DEAE-Sepharose CL-6B, hydroxylapatite and Sepharose CL-6B. The enzyme reduces different alpha,beta-unsaturated carboxylate anions such as (E)-2-butenoate, (E)-2-methyl-2-butenoate, (E)-cinnamate and probably many others in a NADH-dependent reaction to the saturated carboxylate anions. Fumarate, 3-phenyl-2-propinate, 2-enoyl-methyl and CoA esters proved not to be substrates for the purified reductase. NADPH does not act as an electron donor. The enzyme was shown to have a molecular weight of about 450,000 by gel chromatography. It consists of subunits with a molecular weight of 78,000. Per subunit about 1 FAD, 3.5--3.8 atoms of iron and 4.0 labile sulphur atoms have been found indicating a conjugated iron-sulphur flavo-protein. Copper could not be detected. The isoelectric point was 8.4. As shown by absorption spectroscopy the enzyme can be reduced by NADH and reoxidized with dichloroindophenol, hexacyanoferrate III, oxygen and substrates. Addition of 8 mol p-hydroxymercuribenzoate to 1 mol subunit completely destroyed the activity of the reductase. So far no physiological role of the enzyme is known.

Role of ammonium ion in the biosynthesis of beta-nitropropionic acid

The metabolism of inorganic nitrogen compounds was studied in extracts of Penicillium atrovenetum which had been grown under conditions in which beta-nitropropionic acid (BNP) synthesis varied from 0 to 12.5 mumoles per ml. None of the extracts was able to oxidize ammonium ion or nitrite. An enzyme was detected which catalyzed the oxidation of hydroxylamine with cytochrome c as the electron acceptor. The activity of this enzyme was not related to the ability of the organism to produce BNP. Nitrate and nitrite reductase activities were detected only in P. atrovenetum cultures grown on nitrate as a nitrogen source. These results indicated that BNP synthesis is probably not directly associated with the metabolism of inorganic nitrogen compounds and that an organic pathway for the formation of the nitro group is more likely. The activities of certain enzymes related to the metabolism of aspartic acid were investigated. Aspartate ammonia-lyase activity could not be detected in P. atrovenetum extracts. Aspartate aminotransferase and glutamate dehydrogenase activities were found in the extracts but were highest in the cultures which did not produce BNP. beta-Nitroacrylic acid reductase activity was highest in extracts of cultures which were actively synthesizing BNP.