Labelled compounds are compounds in which one or more of the atoms of a portion of the molecules is replaced with or substituted by a detectable quantity of stable or radioactive isotope(s) of the elements present in them chemically, enzymatically or biosynthetically, using suitable precursors, reagents, reaction schemes and procedures. The compounds can be labelled at the specific positions (1,2 or ?, ?, etc.) or generally labelled (denoted by 'G') or uniformly labelled (denoted by 'U').

The isotopically labelled compounds have numerous and varied applications as tracers. They are versatile research tools, which can be advantageously employed in studies pertaining to chemistry, biology, medicine, agriculture, biotechnology and genetic engineering. Of late, there has been an upsurge of interest and activities amongst researchers, especially in Life Sciences to study the various fundamental phenomena, hitherto not unravelled fully, involving in the synthesis, uptake, fate and function of amino acids, polypeptides, proteins, nucleic acids and their components (DNA & RNA), fatty acids (lipids), carbohydrates (sugars), prostaglandins, steroids, pheromones, neurotoxins, anti-aging compounds and a multitude of various other compounds of interest. For obtaining metabolic and pharmaco kinetic data, a widely used method, often the method of choice, is that of radio labelling the compound to be studied. This is mainly due to the fact that pico moles to femto moles of a substance in question can be detected and quantitated using this technique with comparative ease and precision. The tables 1 & 2 show some of the important applications of labelled compounds and labelled biomolecules in different disciplines.

Nucleotides are the basic unit of nucleic acids just as aminoacids are the basic unit of proteins and monosaccharides (carbohydrates) are of polysaccharides (sugars). Nucleotides are obtained on hydrolysis of deoxyribonucleic acid (DNA) and ribonucleic acids (RNA) under controlled conditions. They essentially contain a phosphate group, ribose or deoxyribose (pentose) sugar and purine (adenine or guanine) or pyrimidine (Cytosine, uracil or thymine) nitrogenous base attached together in a specific well defined way. The five important ribonucleotides are adenylic acid, guanylic acid, uridylic acid, cytidylic acid and thymidylic acid.

A heterocyclic nitrogenous base- a purine (adenine or guanine) or pyrimidine (cytosine, uracil or thymine ) in N-glycoside linkage with a pentose ( ribose or deoxyribose ) sugar is called a nucleoside. The five important ribonucleosides are adenosine, guanosine, uridine, cytidine and thymidine. In short, a base linked to a sugar is called a 'nucleoside' and when a phosphate group is added, the base- sugar-phosphate is called a 'nucleotide'.

Molecular biology 'cold' kits (as they do not contain any radioactive material) are ready-made kits, which contain all the materials; chemicals and biochemical components required for readily carrying out the specified reaction for number of experiments. They can be employed in conjunction with radio-labelled products (to obtain radiolabelled DNA / RNA probes) or with non-radioacive products (to obtain fluorescent or chemiluminescent labeled DNA /RNA probes). e.g. using a Nick translation kit in conjunction with a ? labelled 32P or 33P deoxyribo nucleotide, one can introduce radioactively labelled nucleotides into DNA 'in vitro'.

Synthetic oligonucleotides are single-stranded DNA fragments assembled from mononucleotide units by chemical synthesis according to a defined sequence and are generally 20-30 base long (i.e. low molecular weight). The base sequence is easily programmed in the computer of the gene assembler machine, which then determines the addition of reactants in the desired order. Synthetic oligonucleotides (DNA primers) are widely used in the study of genes, especially by researchers of 32P and 35S biomolecules in reactions such as DNA sequencing, polymerase chain reaction (PCR), etc.

'Permaluci' nucleotide formulation contains a secondary stabilizer, which helps in restoring the stability of the 32P & 33P labelled nucleotides at ambient temperatures and a red-coloured dye to impart the dispensed sample solution better visibility.