Structurally characterized one oxo–desoxo bridged Mo2–bis(dithiolene) complex and its interconversion to a discrete oxo or desoxo DMSOR model

The dissymmetric binuclear complex 1 acts as a precursor of the molybdoenzyme models of the dimethylsulfoxide reductase (DMSOR) class.

Side Effects of Nitrification Inhibitors on Non Target Microbial Processes in Soils

Agricultural chemicals have been used extensively in modern agriculture and toxicological studies suggest a great potential for inducing undesirable effects on non target organisms. A model experiment was conducted in order to determine side effects of three nitrification inhibitors (NIs, 3,4dimethylpyrazolephosphate = DMPP, 4-Chlormethylpyrazole phosphate = ClMPP and dicyandiamide = DCD) on non target microbial processes in soils. Side effects and dose response curve of three NIs were quantified under laboratory conditions using silty clay, loam anda sandy soils. Dehydrogenase, dimethylsulfoxide reductase as well as nitrogenase activity (NA) and potential denitrification capacity were measured as common and specific non target microbial processes. The influence of 5-1000 times the base concentration, dose response curves were examined, and no observable effect level = NOEL, as well as effective dose ED10 and ED50 (10% and 50% inhibition) were calculated. The NOEL for microbial non target processes were about 30–70 times higher than base concentration in all investigated soils. The potential denitrification capacity revealed to be the most sensitive parameter. ClMPP exhibited the strongest influence on the non target microbial processes in the three soils. The NOEL, ED10 and ED50 values were higher in clay than in loamy or sandy soil. The NIs was the most effective in sandy soils.Keywords: microbial non target processes, nitrification inhibitors, soil enzymes

Oxidoreductases

Oxidoreductases constitute a very large class of enzymes. They are dehydrogenases and reductases that catalyze the removal or addition of the elements of molecular hydrogen to or from substrates. Enzymatic dehydrogenation is sometimes linked to auxiliary functions such as decarboxylation, deamination, or dehydration of the substrate, as in the actions of isocitrate dehydrogenase (decarboxylation), glutamate dehydrogenase (deamination), and ribonucleotide reductase (deoxygenation). The best known oxidoreductases are the NAD-dependent dehydrogenases, and a thorough discussion of the actions of these enzymes could easily fill a volume the size of this book. For this reason, this discussion must focus on the salient aspects of reaction mechanisms that represent the major classes of oxidoreductases. Authoritative reviews on the kinetics and structures of the main dehydrogenases are available (Banaszak et al., 1975; Brändén et al., 1975; Dalziel, 1975; Harris and Waters, 1976; Holbrook et al., 1975; Rossman et al., 1975; Smith et al., 1975; Williams, 1976). In this chapter, we emphasize the diverse oxidoreduction mechanisms and place less emphasis on auxiliary functions such as decarboxylation, the mechanisms of which are similar to the actions of enzymes discussed in earlier chapters of this book. Discussions of several dehydrogenases not included in this chapter can be found in other chapters. These include methanol, glucose, and methylamine dehydrogenases in chapter 3, dimethylsulfoxide reductase in chapter 4, and dihydrofolate reductase and β-hydroxymethylglutaryl CoA reductase in chapter 5. Pyruvate and α-ketoglutarate dehydrogenases are discussed in chapter 18. Enzymatic addition or removal of the elements of hydrogen to or from an organic molecule generally requires the action of a coenzyme. In principle, the process may proceed by any of several mechanisms, including the formal transfer of a hydride and a proton; or the transfer of two electrons and two protons; or the transfer of a hydrogen atom, an electron, and a proton; or any of several other sequences. Proteins alone do not efficiently catalyze these processes; coenzymes and cofactors generally provide the essential chemistry for catalysis by oxidoreductases. Many enzymes catalyze the dehydrogenation of an alcoholic group to a ketone or aldehyde coupled with the reduction of NAD+ to NADH.