The exchange of nitric oxide in nitrosylmyoglobin, the heme pigment of nitrite-cured meat, has been studied using nitrogen-15 labelling in aqueous solution under conditions (pH, concentration of ascorbate and nitrite) similar to those prevailing in meat during the curing process, and has been found to have a half-life of approximately 2 h at 40 degrees C. One nitric oxide molecule is coordinated to the iron(II) centre of a myoglobin molecule and, in weakly acidic aqueous solution under anaerobic conditions, the exchange rate of the bound nitric oxide is proportional to the concentration of nitrosylmyoglobin, nitrite and hydrogen ion. The rate of exchange has a moderate temperature dependence, corresponding to an activation barrier of delta H+- = 47 +/- 3 kJ.mol-1 at 25 degrees C and pH 5.9, a value dramatically lower than that found for the enthalpy of activation for the oxidation of nitrosylmyoglobin by molecular oxygen, delta H+- = 110 kJ.mol-1. The difference in temperature dependence between the exchange and the autoxidation is discussed in relation to the function of nitrosylmyoglobin as antioxidant in cured meat products.
Suspensions of two strains of Pseudomonas aeruginosa (ON12 and ON12-1) were used to reduce N03and N02-, respectively, to N20. The evolved N20 was quantified by gas chromatography with electron capture detection, and the 15N abundance was determined by mass spectrometry with a special inlet system and triple-collector detection. Sample gas containing unknown N20 pools as small as 0.5 ng of N was analyzed by use of a spike technique, in which a reference gas of N20 of natural '5N abundance was added to obtain enough total N for the mass spectrometer. In N03or N02pools, the 15N abundance could be determined in samples as small as approximately 3.5 ng of N. No cross-contamination took place between the N03and N02-pools. The excellent separation of N03and N02pools, small sample size required, and low contamination risk during N20 analysis offer great advantages in isotope studies of inorganic N transformations by, e.g., nitrifying or denitrifying bacteria in the environment. The great interest in inorganic nitrogen cycling in both aquatic and terrestrial environments has resulted in development of numerous techniques to study the microbial processes involved. These include specific bioassay techniques to extract and quantify the pools of NO3and NO,-(4, 6). In heterogeneous soil environments, it is important to be able to analyze very small subsamples of soil and small pools of NO3and NO2-, which are involved in plant uptake and microbial nitrification and denitrification. NO,-is an intermediate in both processes and is therefore an important link between them. It has been reported that accumulation of NOand NO2can occur in the rhizosphere (4), but whether the NO,accumulation results from nitrification or denitrification activity is not yet understood.
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