ISSN:1025-6016    

DOI:10.1080/10256019708036326

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Isotope effects represent perhaps one of the most versatile tools available to investigators interested in the determination of reaction mechanism, particularly in the case of the mechanistic enzymologist. Interpretation of isotope effect data is somewhat more difficult for enzyme reactions, since the chemical or isotope-dependent step(s) is(are) normally not solely rate-limiting as they are for non-enzyme-catalyzed reactions. One can, however, take advantage of rate-limitation by multiple steps in an enzyme-catalyzed reaction to obtain information on a number of aspects of mechanism. In this paper, simple theory for the application of isotope effects to reaction mechanism is developed, and applied to organic reactions and those catalyzed by enzymes. Techniques used to measure isotope effects depend somewhat on the isotope used, that is radioisotopevs.stable isotope, or hydrogen isotopevs.heavier atoms. Techniques to be discussed include competitive and noncompetitive (or internal discrimination) measurements. In enzymecatalyzed reactions, information can be obtained on the order of addition of reactants and release of products, and this will be illustrated using the 6-phosphogluconate and alcohol dehydrogenase reactions. The use of multiple isotope effects can be used to distinguish between stepwise and concerted reactions, and this will be illustrated with the formate and glucose 6-phosphate dehydrogenase and malic enzyme reactions.

ISSN:1025-6016    

DOI:10.1080/10256019708036327

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Inter- and intra-molecular non-statistical isotope distributions do not only require the existence of a kinetic isotope effect on a defined enzyme catalyzed reaction, but also the prerequisite that this reaction is located at a metabolic branching point. Furthermore a metabolic and isotopic balance demand that the extent of the isotopic shift is reciprocal to the products' yields. On this base the13C-enrichment of L-ascorbic acid in position C-1 and the depletion of glycerol in C-1 are interpreted. The13C-pattern of natural malic acid is discussed as a consequence of isotope effects on the carboxylation of pyruvate and PEP and on the pyruvate dehydrogenase reaction. The patterns of natural products synthezised by transfer of “active acetaldehyde” is proposed to be due to an isotope effect on the thiamine pyrophosphate containing lyase reaction. An isotope effect on the reduction of “active formaldehyde” to “active methyl” and the existence of corresponding pools is responsible for13C-enrichments and depletions of natural products in positions bearing these intermediates. Finally a model for the main nitrogen pools and for isotope discriminations between α-amino, ω-amino-N and amide pools in plants is proposed.

ISSN:1025-6016    

DOI:10.1080/10256019708036328

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In this contribution, the various and fundamentally different uses and applications of isotope signatures (of both heavy and light elements) are discussed. Examples are given for the successful use of87Sr/86Sr in uncovering fraud in wine trade. Also, in an experiment related to “Waldsterben”,87Sr/86Sr analyses reveal rather unexpected responses of spruces and maple-trees to mechanical damage of their roots. In another study, from206Pb/207Pb (and208Pb/207Pb) analyses of tree growth-rings it is demonstrated that they do not accurately record lead burdens in the environment. This is contrary to current views on the subject of tree-rings as reliable banks of past heavy metal pollutions of the biosphere. Furthermore, new perspectives of applications of isotopes in biological tissues, including those of cosmogenic and nucleogenic origin will be shortly outlined.

ISSN:1025-6016    

DOI:10.1080/10256019708036329

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Locust nymphs were raised from hatching to adult locusts on either seedling wheat (C3) or maize (C4), to determine whether relative enrichments/depletions of15N and13C within body tissues are influenced by diet. The maize contained less hexose sugars and protein per gram than wheat.

ISSN:1025-6016    

DOI:10.1080/10256019708036330

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:1025-6016    

DOI:10.1080/10256019708036331

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Sulfur isotope ratios of mangrove leaves of 19 species were compared to discuss the species-specific characteristics of sulfur uptake and assimilation. The members ofRhizophoraandBruguieraalways show remarkable enrichments of the light isotope, giving negative δ34S values in most cases. The elaborated root systems of such species seem to be closely related to their sulfur absorbing systems as an adaptation to their anaerobic soil conditions.

ISSN:1025-6016    

DOI:10.1080/10256019708036332

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The dinitrogen fixation activity ofAzospirillum sp., andPantoea agglomeransstrains was determined by15N2incorporation after incubation with15N2labeled air or/and by acetylene reduction. These bacterial strains were able to fix N2both in pure culture and in association with wheat plants in hydroponics. Nitrogenase activity ofAzospirillum sp., in pure culture was more rapidly inhibited by the addition of NH4+than NO3−. The N2fixation ofP. agglomeransdecreased only by NH4+-addition, but was stimulated by NO3−. Nitrogen fixation in association with wheat plants remained unaffected by both N compounds. However, nitrogen derived from the atmosphere (Ndfa) contributed only very little to the overall nitrogen nutrition of the plants.

ISSN:1025-6016    

DOI:10.1080/10256019708036333

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Naturally produced methane shows different δ13C-values with respect to its origin,e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane—given in literature—range between −80‰ and -50‰ and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gasesviaimplanted fistulae we compared δPDB13C-values of methane and CO2between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n= 47) we calculated δPDB13C-medians resulting in small but not significant differences within and significant differences between the species for CO2and methane. The δPDB13C-differences between methane and CO2were statistically equal within and also between the species. Therefore a linear regression of methane values on CO2is appropriate and leads to: δPDB13C(methane)‰ = 1,57 * δPDB13C(CO2)‰-47‰ with a correlation coefficient ofr= 0,87.

ISSN:1025-6016    

DOI:10.1080/10256019708036334

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We report the first isotopic study of an animal host-parasite system. Parasitic, intestinal nematodes,Graphidium strigosumandPassalurus ambiguus, were15N-enriched relative to their host, the European rabbitOryctolagus cuniculus, while parasitic cestodes,Cittataenia denticulataandMosgovoyia pectinata, were15N-depleted, suggesting different trophic relationships. Host embryos were more similar in their δ13C and δ15N values to maternal muscle than were any of the parasites. Coprophagy, the direct recycling of food by the rabbit eating its own faeces, did not lead to isotopic differences between stomach contents and faeces, suggesting that the major point for isotopic discrimination in lagomorph nitrogen metabolism is in the animal rather than in the gut. We conclude that bulk δ13C and δ15N can reveal valuable new information about host-parasite relationships, and these could be explored further at the biochemical level using compound-specific isotopic analyses.

ISSN:1025-6016    

DOI:10.1080/10256019708036335

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor