Isotopic Labeling
Introduction
Isotopic labeling is a technique used in chemistry and biochemistry to track the passage of an isotope, an atom with a detectable variation in neutron count, through a reaction, metabolic pathway, or cell. The reactant is 'labeled' by replacing specific atoms by their isotope. The reactant is then allowed to undergo the reaction. The position of the isotopes in the products is measured to determine the sequence the isotopic atom followed in the reaction or the cell's metabolic pathway. The labeled atom, or isotopically labeled atom (often a labeled carbon, nitrogen, hydrogen, or oxygen atom), is called a radioactive tracer.
History
The first isotopic labeling experiment was carried out by Harold C. Urey in 1931. He was studying the reaction of molecular hydrogen with oxygen to produce water. Urey used heavy water, which contains the heavy isotope of hydrogen, deuterium (D), to label the hydrogen molecules. This allowed him to observe the reaction pathway and determine the reaction mechanism.
Types of Isotopic Labeling
There are several types of isotopic labeling, including radioactive and nonradioactive isotopic labeling.
Radioactive Isotopic Labeling
In radioactive isotopic labeling, radioactive isotopes are used. These isotopes decay and emit radiation, which can be detected and measured. Radioactive isotopes commonly used in biological systems include carbon-14 (14C), hydrogen-3 (tritium, 3H), sulfur-35 (35S), phosphorus-32 (32P), and iodine-125 (125I).
Nonradioactive Isotopic Labeling
Nonradioactive isotopic labeling involves the use of stable (nonradioactive) isotopes. These isotopes do not decay and therefore do not emit radiation. Instead, their presence is detected by the difference in mass compared to the more common isotopes. Stable isotopes commonly used in biological systems include carbon-13 (13C), nitrogen-15 (15N), and deuterium (2H or D).
Applications
Isotopic labeling has a wide range of applications in various fields such as chemistry, biochemistry, molecular biology, and medicine.
In Chemistry
In chemistry, isotopic labeling is used to understand the mechanism of chemical reactions. By labeling one of the reactants, it is possible to track the position of the atoms through the reaction pathway.
In Biochemistry and Molecular Biology
In biochemistry and molecular biology, isotopic labeling is used to trace the path of carbon in photosynthesis, the flow of nitrogen in nitrogen fixation, the uptake of water in hydration reactions, and the metabolic pathways of various biochemical reactions.
In Medicine
In medicine, isotopic labeling is used in diagnostic techniques such as PET scans. Radioactive isotopes are used to label molecules of interest, such as glucose in glucose metabolism studies. The labeled molecules are then tracked in the body using the emitted radiation.
Advantages and Disadvantages
Like any scientific technique, isotopic labeling has its advantages and disadvantages.
Advantages
The main advantage of isotopic labeling is that it allows for the tracking of atoms through reactions and metabolic pathways. This can provide valuable information about the mechanism of these processes. Additionally, because the isotopes are chemically identical to their non-isotopic counterparts, they do not alter the reaction or pathway.
Disadvantages
The main disadvantage of isotopic labeling is the potential for radiation damage when radioactive isotopes are used. This can alter the reaction or pathway and lead to inaccurate results. Additionally, the detection of isotopes can be challenging and require specialized equipment.