For the study of biological pathway and mechanism Tracer techniques has its importance. Tracer techniques involve use of isotopically labelled molecules and detection of the isotopes for the study.
An isotope is a form of an element which has the same atomic number of electrons as the common form of the element but it differs in atomic weight. The difference of atomic weight is due to difference of number of neutrons in its nucleus. An isotope may be stable or radioactive depending on the relative number of protons and neutrons in its nucleus.
Both stable and radioactive isotopes of an element are identical in chemical properties, and thus they undergo all the physical and chemical changes like the ordinary form of the element. Moreover, the can be detected at any time by atomic weight or radioactivity. By Ginger-Muller counter or other sensitive detectors the radioactive isotopes can be detected by their radioactivity even when it present in very small quantity. Detection of stable isotopes can be done by their atomic weight through a mass spectrograph. Stable or radioactive isotopes used for studying the fate of a molecule in physical, chemical or biological processes are called tracer element and the methods for such studies are called tracer techniques . Commonly used radioactive tracers in the study of biology are C14, P12, H3 etc. An important stable isotope used as tracer in biology is O18.
Tracer Technique
At specific positions in their molecules inorganic and organic compounds can be prepared with isotopes. Such compound containing an isotope in its molecule is used as a tracer. In the tracer technique the isotope element in the molecule is said ‘tagged ‘or ‘labelled’. When an isotopically labelled compound is administrated to an animal or a plant or incubated with tissue preparations, it undergoes same fate as the unlabelled form of the compound and the isotopically labelled products can be detected. In this way, this source, metabolic pathway and end products of bio-molecules can be studied with the use of isotopically tagged tracers.
In the process of Tracer techniques tracers are also used for determining the following: –
i) Metabolic turnover of a substance.
ii) Relative proportion of a substance being catabolised through different pathways.
iii) Intestinal absorption of the nutrient.
iv) Volume of body fluids.
v) Blood level of a hormone.
vi) Mechanism and site of action of a hormone.
vii) Cardiac output.
viii) Flow of blood through an organ.
ix) Intracellular distribution, i.e. autoradiography.
The most common radioactive tracer used in metabolic studies is C14. P32, which is a radioactive tracer is mainly used to study the phosphorylation reactions. H3 or tritium is also a radioactive isotope which is used as a tracer in the form of tritium oxide (THO) for determination of total body fluid volume. O18 is a stable isotope and it is used to trace the source of O2 liberated in photosynthesis. When O18 labelled water (H2O18) is used in photosynthesis, O182 is liberated. It is thus proved that the water is the source of oxygen liberated in photosynthesis.