Metabolite Pools and Metabolic Branching as Factors of in-vivo Isotope Discriminations by Kinetic Isotope Effects

Isotopes in Environmental and Health Studies

Robins

Werner

2015

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Section: Kinetic Isotope Effects On Enzyme-catalysed Reactions and Pamentioning

confidence: 99%

Multi-factorialin vivostable isotope fractionation: causes, correlations, consequences and applications

Isotopes in Environmental and Health Studies

Robins

Werner

2015

“…These correlations are connected to molecular structures and reaction mechanisms and they are basically also the cause for intermolecular isotope discriminations. We have studied the influences of in vivo conditions on kinetic isotope effects and isotope patterns of carbon (Schmidt et al 1995;Schmidt and Gleixner 1998;Schmidt and Kexel 1998;Schmidt 1999), oxygen , nitrogen (Werner and Schmidt 2002) and hydrogen (Schmidt and Eisenreich 2001;Schmidt et al 2003) in natural compounds. These basic analyses have provided the possibility of summarising and combining all available essential aspects of in vivo isotopic discriminations.…”

Section: Introductionmentioning

confidence: 99%

Fundamentals and systematics of the non-statistical distributions of isotopes in natural compounds

2003

Naturwissenschaften

112

The intermolecular and intramolecular non-statistical distribution of the isotopes of the bio-elements in natural compounds must obviously be controlled by logical principles. However, a critical review of the available isotope patterns of natural compounds indicates that a previously discussed general thermodynamic order and its mechanistic foundation cannot satisfactorily explain all experimental data. In the present contribution it is shown that a partial thermodynamic order can eventually be attained for defined positions and compounds under steady-state conditions of metabolism. However, as biological systems are generally open and irreversible, many other in vivo isotope discriminations are dominated by kinetic isotope effects, even in context with reversible reactions. On the other hand, kinetic isotope effects can only become effective in vivo in combination with metabolic branching and the implied isotope shifts of the products are balanced by their relative yields. In vivo, the influences of thermodynamic and kinetic isotope effects are modulated by interferences of the actual metabolic conditions, such as the nature and kind of precursors, alternative metabolic pathways, metabolite pools and fluxes, and by reaction mechanisms. This is demonstrated by giving examples for the isotopes of hydrogen, carbon, nitrogen, oxygen and sulfur, while simultaneously the particularities of the individual elements are elaborated. The resulting general theory of the origin of non-statistical isotope distributions in biological systems permits the interpretation and prediction of isotope patterns and provides the scientific basis for the elucidation of biosyntheses and origin assignments of natural compounds.

“…On the other hand, isotope discriminations must be observed, when reactions implying kinetic isotope effects are connected to metabolic branching, and the products -as demanded by an isotopic balance -must show depletions and enrichments reversely proportional to their relative yields. This is observed with many examples of metabolically related products [2]. Finally, a reaction sequence under conditions of a steady state, proceeding closely to equilibrium, and implying a considerable loss of substrate or intermediates, will be accompained by a remarkable isotope discrimination, because it is basing on the accumulation of discriminations by kinetic and equilibrium isotope effects of several steps.…”

Section: Introductionmentioning

confidence: 92%

“…Bacterial nitrification and denitrification or NH3-evaporation are accompanied by large isotope discriminations [8], while reactions within the plant, even when they imply kinetic isotope effects like the assimilatory nitrate reduction or the glutamate synthetase reaction, do not largely contribute to isotope discriminations, because they are undirectional with total consumption of their substrate [9]. Nevertheless, so far unknown reactions or processes contribute to the creation of pools with different "N-content like the a-amino pool, the amide or the heteroaromatic N-pool and the w-amino pool [2]. This is so far of a quite limited importance in regard to an origin assignment; here in most cases only the global 6''N-value is used for obtaining hints on local conditions of fertilization.…”

Section: Nitrogen Isotopes Influences Of the "Open System" Soil And mentioning

confidence: 99%

“…Normally the global isotope abundance of natural compounds is correlated to that of the primary inorganic sources (C02, HzO, 02, N2), while isotopic patterns are caused by isotope effects on defined reactions during the biosynthesis. However, in complex systems like plants, the manifestation of isotope effects as isotope discriminations is depending on certain prerequisites of the metabolism; most important are the degree of reversibility of reaction sequences involved and the existence of branching points, compartments and pools in biological systems [2] (Fig.1).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Isotope Discriminations Upon Biosynthesis in Natural Systems: General Causes and Individual Factors of the Different Bioelements

Isotopes in Environmental and Health Studies

1999

The global isotope abundance of natural organic compounds is determined by that of their precursors. A prerequisite for the formation of non-statistical intermolecular and intramolecular isotope distributions is the occurance of thermodynamic and kinetic isotope effects in irreversible and branched metabolic processes. This coincidence permits the interference of external factors like climatological conditions during biosynthesis, thus leading to corresponding modulations of isotope fractionations. The general principles of the in-vivo isotope discrimination are outlined and the individual properties of the different bioelements in this system are described. The results are discussed in regard to obtain maximum information about the origin and authenticity of a natural compound by multielement isotope analysis.