Nitrogen turnover rate

Slawyk, G., CoUos, Y., and Auclair, J. (1977). The use ofthe Cand N isotopes for the simultaneous measurement of carbon and nitrogen turnover rates in marine phytoplankton. Limnol. Oceanogr. 925-932. 

Our results do not support the protein stress model. However, this model may apply in cases where stress is intermittent and results in tissue loss, as observed in the study of crows (Hobson and Clark 1992). Low protein levels throughout life after weaning may have produced overall slow and reduced rate of growth rather than tissue loss. Adult rats fed protein-deficient diets after maturation show systematic losses of nitrogen from most tissues that are in proportion to their turnover rates and masses (Uezu et al. 1983). Perhaps tissue nitrogen isotope enrichment may occur under these conditions. New experiments are needed to evaluate this hypothesis. 

Amino Acids Amino acids that enter the liver follow several important metabolic routes (Fig. 23-14). (1) They are precursors for protein synthesis, a process discussed in Chapter 27. The liver constantly renews its own proteins, which have a relatively high turnover rate (average half-life of only a few days), and is also the site of biosynthesis of most plasma proteins. (2) Alternatively amino acids pass in the bloodstream to other organs, to be used in the synthesis of tissue proteins. (3) Other amino acids are precursors in the biosynthesis of nucleotides, hormones, and other nitrogenous compounds in the liver and other tissues. 

Labeling of the phytoplankton nitrogen pool in this linear fashion was contrary to the expected result. The solid line in Figure 4 shows the expected asymptotic increase in 15N that was calculated by using the DIN concentrations in the epilimnion, the addition rate of 15NH4, and the assumption of a 1-week nitrogen-turnover time (87) in the phytoplankton. A similar asymptotic increase in 15N content of phytoplankton, with the phytoplankton reaching isotopic equilibrium in 1 week, was measured in an isotope-addition experiment done in limnocorrals in nearby Toolik Lake (Kipphut, G. W. Whalen, S. C. unpublished data). 

Table 1.2 Oceanie Inventory, Turnover Rates, and Residence Times for the Major Fixed Nitrogen Species in the Ocean...

The rate used here is what has been called the nominal turnover rate (4), moles reacted per second per mole of sites, taken as surface atoms for metal catalysts. These sites are measured, for example, by hydrogen chemisorption, electron microscopy, X-ray diffraction, and magnetic methods. Of course, only a fraction of these sites may be active, but this fraction has to be learned by kinetic experiments and is subject to change as new kinetic results become available (6). This fraction seems to be known for ammonia synthesis over iron and can be measured by nitrogen chemisorption. 

As the mean C/N ratios are rather constant for the stations covered by the long-term measurements (Fig. 12.12), one may also assume that similar estimates can also be made for DON. Unfortunately, due to the higher turnover rate of DON compared to that of DOC and additional processes, which affect the total amount of nitrogen in the system, similar calculations cannot be done for DON with the data available. 

Fig. 4.14. Comparison of CO oxidation activities of palladium-on-zirconia catalysts prepared by in situ activation from amorphous Pd33Zr(>7 ( ) and by conventional impregnation of zir-conia with a palladium salt (O). Arrhenius plots of the turnover frequencies are plotted. Conditions reactant gas mixture. 1700 ppm of CO, and 1700 ppm 03 in nitrogen flow rate. 150 ml (STP) min amount of catalyst, 0.37 g O 1.24 g...

Reactions of hydrazine (N2Hs ), phenylhydrazine (N2H4Ph ) at pH 2.8 and azide (Nj-) at pH 5.3 with [Ru (HL)(H20)] and [Ru "L(H20)] (where L = ethylenediamminetetraacetate) were studied by voltammetry and spectrophotometry at 25 °C. The resultant complexes were electrolyzed in the presence of nitrogenous ligands in excess by constant potential coulometry at Hg-pool electrode. The turnover rates for the formation of ammonia and/or amine and the catalytic efficiency of these nitrogenous compounds were reported. An appropriate mechanism for the catalytic reduction of hydrazines and azide were proposed. 

Table 9-2. Mass Content of NHy and NH% Nitrogen in the Troposphere and Turnover Rate Resulting from Known Residence Times t ...

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