Stable heavy isotopes

Another widely used approach to the elucidation of metabolic sequences is to feed cells a substrate or metabolic intermediate labeled with a particular isotopic form of an element that can be traced. Two sorts of isotopes are useful in this regard radioactive isotopes, such as and stable heavy isotopes, such as or (Table 18.3). Because the chemical behavior of isotopically labeled compounds is rarely distinguishable from that of their unlabeled counterparts, isotopes provide reliable tags for observing metabolic changes. The metabolic fate of a radioactively labeled substance can be traced by determining the presence and position of the radioactive atoms in intermediates derived from the labeled compound (Figure 18.13). 

Stable heavy isotope nitrogen ( N) is determined using either mass spectrometry or emission spectrometry (Fiedler and Proksch 1975, Hauck 1982). Radioactive is measured by direct emission measurements or by liquid scintillation methods (Tiedje 1981). 

TABLE 7.2 Natural Abundance of Stable Heavy Isotopes... 

O is a stable, heavy isotope of normal oxygen which is, itself, a mixture of three stable isotopes, namely. O, O, and O. Fatty acids with O or O in the carboxyl group have not yet been employed in biological studies. O has, however, been used in the study of photosynthesis and in hydrolysis of esters. Each of the radioactive isotopes of oxygen (O, O, O ) has too short a half life to be of use in metabolic studies. 

The usual method of study is to suggest a possible precursor and to feed it to the biosynthesizing system. The precursor has to be labelled in some way to trace it through the sequence of reactions, and that is usually by some isotopic element. It may be a radio-active isotope, such as H, " 0, or that can be followed by its radiation or it can be a stable heavy isotope, such as H, C, N, or 0, that can be traced by mass spectrometry or nuclear magnetic resonance (NMR) spectroscopy (Table 5.1). Another possible way is to use mutant strains of an organism that lack the enzymes to complete a particular synthesis, or to add a specific enzyme inhibitor, so that intermediates accumulate and can be identified. A mutant strain of yeast was important in discovering mevalonic acid and its place in terpene biosynthesis (Chapter 6) and a number of mutants of the bacterium Escherichia coli helped to understand the shikimic acid pathway (Chapter 8). 

The discovery of the isotopes oxygen-17 and oxygen-18 by Giauque and Johnston in 1929 led to the confirmation of the existence of a stable heavy isotope of hydrogen and eventually to the discovery of the stable isotopes of carbon and nitrogen. 

The homogeneous reactor experiment-2 (HRE-2) was tested as a power-breeder in the late 1950s. The core contained highly enriched uranyl sulfate in heavy water and the reflector contained a slurry of thorium oxide [1314-20-1J, Th02, in D2O. The reactor thus produced fissile uranium-233 by absorption of neutrons in thorium-232 [7440-29-1J, the essentially stable single isotope of thorium. Local deposits of uranium caused reactivity excursions and intense sources of heat that melted holes in the container (18), and the project was terrninated. 

Properties of Radioactive and Stable as Tracers in Metabolic Studies Heavy Isotopes Used... 

As you can see from Figure 2.5, the neutron-to-proton ratio required for stability varies with atomic number. For light elements (Z < 20), this ratio is close to 1. For example, the isotopes C, N, and are stable. As atomic number increases, the ratio increases the belt of stability shifts to higher numbers of neutrons. With very heavy isotopes such as 2j Pb, the stable neutron-to-proton ratio is about 1.5 ... 

The global atmospheric circulation acts as an enormous filtration system, which depletes high-latitude precipitation of heavy isotope-bearing water molecules. Because of this system, measurements of the stable isotopic composition of the ice sheets and of ocean-floor sediments reveal very important paleo-environmental information (see Sections 18.2.2,18.3.2, and 18.3.3). Here we examine this filtration system at a physical level. This system was first understood by a great Danish geochemist named Willi Dansgaard (Dansgaard, 1964). 

Radioactive decay is a nuclear process from an intrinsically unstable nucleus that emits alpha particles, beta particles and gamma rays. The loss of mass from the nucleus changes the element to one of a lower mass. Carbon dating uses the decay of the 14C nucleus, a heavy and unstable isotope of carbon, to become the stable 14N isotope. The overall process is written ... 

Any of these data banks, those parts from the ice ages, can have their stable isotope ratios perturbed by the huge ice reserves which were removed from the sea and piled up on land, because the ice depletes the oceans in the light isotopes, and therefore significantly enriches the sea in the heavy isotopes, so that sea sediments and continental precipitation, rain and snow, reflect this perturbation as well as perturbations caused by temperature changes alone. 

Using the above laboratory relations we compute the values of 6D and 618 expected for rain made by a single distillation from the ocean and for rain made by a subsequent distillation for sea rain which fell on the land. These values show that when rain and snow exhibit very large depletions of the heavy isotopes, e.g., 6D of -200 and -300 ppt, two or three distillations have occurred. It is known from measurements of tritium in rain [76] that two or three distillations occur in the U.S. between evaporation from the Pacific Ocean and precipitation in the eastern U.S. The agreement between the number of distillations deduced from stable isotopes and deduced from tritium is gratifying. 

Each species and phase of an element with two or more stable isotopes consists of light and heavy isotopes in proportions that can be measured by using a mass spectrometer (e.g., Faure, 1986). The isotope ratio R is the quotient of the number of moles of a heavy isotope (such as 18 O) to the number of moles of a light isotope (such as 160). 

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

An alternative approach uses tracers, isotopically enriched materials, which are added to the system studied. Labeled compounds are available with the heavy isotope enriched to a level of 99 atom%. These are more difficult to produce than radioisotopes, making them relatively expensive. Materials labeled with stable isotopes are non-radioactive and pose no hazard to human health or the environment (Barrie Prosser, 2000). 

All elements that form solid, liquid, and gaseous compounds stable over a wide temperature range are likely to have variations in isotopic composition. Generally, the heavy isotope is concentrated in the solid phase in which it is more tightly bound. Heavier isotopes tend to concentrate in molecules in which they are present in the highest oxidation state. 

Figure 1 Studies published between 1988 and 1997 reporting use of secondary ion mass spectrometry (SIMS) in geochemistry and cosmochemistry. Papers are subdivided according to theme Hvy Iso, studies of heavy isotope ratios, principally for U-Pb dating Lt Iso, studies of light stable isotope ratios (H, B, C, O, S) Hvy El, studies primarily focused on analysis of elements >40 amu (e.g., rare earth elements) Lt El, studies primarily focused on analysis of elements <40 amu (e.g., water content) Prec Met, analysis of precious metal contents (e.g., Au, Ag, Pt) Expt, analysis of experimental run products Misc, other mis-cellanous studies utilizing SIMS.

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