Isotope Coded Affinity Tag Biology Essay

Proteomicss is a critical and necessary subdivision of scientific discipline targeted at the in-depth survey of proteins and their construction to understand their map ; as an of import pharmacological tool in drug find and drug development.

The most widely used analytical attack to protein separation and quantification, normally involves incorporating protein separation by 2D polyacrylamide gel cataphoresis with micro capillary contrary phase-liquid chromatography protein designation ; and eventually, sensing by mass spectroscopy.

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However, the presence of restrictions such as the deficiency of mechanization and high costs associated within the combination technique led to the research and debut of a better and more dependable technique affecting the usage of isotope coded affinity tickets ( ICAT ) .

This study looks at the history of isotope coded affinity tickets, its advantages over 2D cataphoretic techniques, the rules associated with the technique, its development over the old ages and eventually its application and part to the growing and development of analytical scientific discipline.

It besides aims to notice on future developmental paths for the engineering.


Section: Section Title Page Number

A. Background 4

B. Introduction to protein quantification 4

B.1. 2D Polyacrylamide Gel Electrophoresis 4

B.2. Reverse Phase High Liquid Chromatography 5

B.3. Mass Spectrometry 5

B.4. Problems associated with 2DLC-MS combination technique 5

C. Introduction to Isotope Coded Affinity Tags ( ICAT ‘s ) 6

C.1. Major promotions in isotope coded affinity ticket attack 6

D Principles of Isotope Coded Affinity Tags ( ICAT ‘s ) 7

D.1. Protein Sampling 8

D.2. ICAT reagent – Taging 8

D.3. Peptide Isolation 8

D.4. Protein quantification 8

D.5. Peptide designation 8

E Applications of Isotope Coded Affinity Tags ( ICAT ‘s ) 9

E.1. Applications in the quantitative designation of malignant neoplastic disease

biomarkers 9

E.2. Applications in the quantification of antimalarial drugs

and their metabolites in biological fluids 9

E.3. Quantification of protein look in oxidative-stressed liver

cells as a curative mark for the intervention of liver disease 10

E.4. Quantitative analysis of defaulted proteins present in the encephalon as

a curative mark for the intervention of encephalon diseases 10

E.5. Applications in the proteomic analysis of recombinant proteins 10

F Future Development of Isotope Coded Affinity Tags ( ICAT ‘s ) 11

G References 12


Proteins are really of import constituents of biologically active systems and some of their maps include structural foundation ( connective tissue ) , transit ( bearer proteins ) or unsusceptibility ( antibodies ) . Specific and selective protein-protein interactions within the organic structure are the footing for cardinal metabolic and kinetic tracts within life beings. A break in a specific protein ‘s interaction and map, taking to a little or big intervention in the subsequent metabolic tract within the organic structure due to any figure of grounds ; is the major cause of disease which if non cover with, can take to human death. For this ground, Proteomics is a critical and necessary subdivision of scientific discipline targeted at the in-depth survey of proteins and their constructions ; to understand their map as an of import pharmacological tool in drug find and drug development. Developments in proteomics and genomics over the old ages through quantitative-structure activity relationship ( QSAR ) surveies and computing machine aided drug design ( CADD ) , has helped to place fresh drugs and their marks for action.


The usage of Isotope coded affinity tickets as a protein quantification method in proteomics was foremost developed in 1999 by Aebersold et al. to help the sensing and purification of recombinant proteins [ 1 ] . Before the research done in 1999, most widely used attacks to protein quantification were done by 2D Polyacrylamide Gel Electrophoresis ( 2D PAGE ) combined with micro-capillary Liquid reversed stage liquid chromatography ( 2DLC ) and fresh electrospray ionisation ( ESI-MS ) or tandem mass spectroscopy ( MS-MS ) technique for sensing [ 2 ] .

Figure 1. “ Conventional representation of the process for analysis of gel-separated proteins by ESI-MS/MS-MS ” [ 2 ]


This is because 2D Polyacrylamide Gel Electrophoresis ( 2D PAGE ) is really good known for its sensitiveness and high deciding separation power. It is besides a extremely adaptable technique, and its resourcefulness makes it extremely kind after for the separation of biological molecules including proteins, based on both physiochemical belongingss and other chemical-specific interactions. The bound of sensing is good documented to a declaration of more than 7000 supermolecules in a remarkable separation. A big assortment and combination of dissolvers and additives can be used with 2D-PAGE cataphoretic technique to guarantee analytes solubility within complex protein mixtures.


The inclusion of liquid chromatography as a 2nd separation measure besides allows for the farther separation of the protein mixtures based on difference in keeping belongingss of the constituents. Recent discovery in the analytical attack to liquid chromatography involves the used of two HPLC pumps connected through a elaborate 6-port valve system ; which consequences in a more comprehensive separation by gradient elution of complex protein mixtures at high velocity and speedy tally times.


Finally, a mass spectrometric technique ( Electrospray ionisation ( EIMS ) or tandem mass spectroscopy ( MSMS ) ) which provides a UV sensing of protein and measures the mass to bear down ratios of the eluted peptides is employed. The sensor produces a comprehensive chromatogram by plotting UV signals against their corresponding contrary stage keeping times, and so the ESI-MS/MS-MS provides mass information for the eluted peptides.

Figure 2: “ The building of a 2DLC column and its interface with mass spectroscopy. ( A ) A force per unit area bomb is used for column wadding and sample burden. ( B ) The flow rate of in the 2-D column is controlled at 100-300 nL/min, and ESI is achieved by using 2 kilovolt to the gold wire. “ [ 4 ; A ; 5 ]


However, in malice of the popularity of the combination technique, a figure of restrictions exist that makes the technique far from perfect. It has been documented that complex proteins and peptides with really high alkalinity or basicity and some trans-membrane proteins can non be separated by this combination method. Besides during entire cell analysis, the combinatorial technique was found to readily suit extremely abundant protein separation with the lower abundant proteins being barely detected. The over procedure besides requires several consecutive phases including hard techniques such as in-gel digestion ; doing the combination technique extremely labour intensifier, hard to automatize and hence non-cost effectual. This called for a farther development in proteomic research to get the better of these jobs by perchance avoiding the separation measure by cataphoresis and therefore the debut of the usage of fresh Isotope coded affinity tickets ( ICAT ) .


The attack of isotope coded affinity labeling chiefly combined with a signifier of high public presentation liquid chromatography and tandem mass spectroscopy ( LC-MS/MS ) is a comparatively new and improved method used in proteomics for the precise quantification and designation of protein sequences within simple or complex protein mixtures. It has been documented to be simpler as it is capable of straight quantifying the proteins from complex mixtures, extinguishing the cataphoretic phase. This makes isotope coded affinity labeling more efficient, easily-automated and hence a batch less labour and cost intensive than the cataphoretic procedure. The usage of ICAT is the new and preferable analytical method for protein quantification.

Isotope coded affinity tagging is based on a category of chemical reagents called ‘Isotope coded affinity tickets ‘ ( ICAT ) . The ICAT reagent occurs in two signifiers depending on the figure of heavy hydrogens ; light incorporating none or heavy incorporating eight. ICAT reagents are made up of three major functional units:

A distinguishable chemically reactive group responsible for the selective labelling of the SH groups of thiol ( cysteine ) residues,

An isotope coded linker responsible for the soluble belongingss of the reagent and it besides provides a site for the add-on of the isotopic label,

And a vitamin H affinity ticket used to accomplish protein isolation and designation. It depends on the rule of strong adhering interaction of vitamin H and avidin.

Figure 3: “ A construction of an ICAT reagent demoing the chemically reactive thiol specific group, a linker and a vitamin H ticket.


Since the technique was ab initio introduced in 1999 for the labelling of protein mixtures at low degrees, there have been valuable technological promotions in the attack utilizing isotope coded affinity tickets ( ICAT ‘s ) within the pharmaceutical industry. These include:

The design and alteration of affinity tickets to better on the chromatographic separation procedure. [ 25 ]

The usage of variable peptide specific affinity tickets to maximize large-scale quantification on single procedures. [ 25 ]

An debut to the combination of different tickets to accomplish maximal proteome industry [ 21 ]

The usage of exopeptidases to expeditiously take the affinity ticket from the peptides in the purification phase [ 22, 23 ]


Isotope coded affinity tickets are used for placing and quantifying the protein content of two different cell provinces or population within a mixture. The technique is based mostly on two constructs:

“ The peptide sequence of the protein to be quantified ( between 5-25 Amino acids long ) contains sufficient information to place that alone protein.

And those peptides tagged with the visible radiation and heavy reagents severally are chemically indistinguishable and therefore service as really ideal internal criterions for quantification. ”

Figure 4. A conventional diagram for the ICAT attack to protein quantification.

The rules of Isotope coded affinity tickets as documented by Aebersold et Al. are divided into four phases: Sampling, Tagging, Isolation and Quantification.

D.1. PROTEIN Sampling

First, two different protein samples incorporating reduced cysteine ( thiol ) side ironss are separately derived ; by interrupting down the cell construction, and insulating and pull outing the proteins required from the cell.


For one of the protein samples, the light signifier of the ICAT reagent ( incorporating zero heavy hydrogen ) is introduced to covalently adhere to the SH cysteine residues ; whilst for the other, the heavy signifier of ICAT reagent ( incorporating eight heavy hydrogen ) is used. The person labelled mixtures represent different cell provinces or populations. The two samples are so combined into one complex protein mixture and a peptidase enzyme is added to ‘cut-up ‘ or split the larger protein molecules into tagged smaller peptides fragments.


Avidin is so introduced to the mixture to move as a “ magnet ” and due to the strong and extremely specific adhering interaction of vitamin H and avidin, the ICAT-tagged peptides are isolated from the mixture through affinity chromatography. The stray peptides are so analysed and separated by micro-capillary high public presentation liquid chromatography- mass spectroscopy ( HPLC-MS/MS ) .


This is the most of import measure of the analytical procedure as the measure and sequence individuality of the proteins from which the tagged peptides originated, are automatically determined.

Quantification is achieved by comparing the integrated extremum strengths for at the same time eluted braces of indistinguishable, double charged peptide ions. The brace corresponds to the two different signifiers of the ICAT reagent with the mass spectrometer running in turn in two manners. One manner measures the comparative fragmenting of peptides eluting from the micro-capillary column whilst the other records the sequence information of the labeled peptides in the same molar ratios as the corresponding proteins. This besides means that the chemically indistinguishable ICAT-labelled peptide ions are readily identified because as they co-elute, they differ in mass-to-charge ( m/z ) ratio because of an 8 heavy hydrogen difference in the mass of the ICAT-reagents.


The concluding phase of isotope coded affinity labeling involves an machine-controlled correlativity with protein sequence informations Bankss utilizing algorithms and substitutions, to place the protein from which the sequenced peptide originated and therefore place the protein. A combination of all consequences generated on the chromatogram by the mass spectrometer ; and analysis of the ICAT reagent-labelled peptides therefore determines the comparative measures every bit good as the sequence individualities of the constituents of protein mixtures in a individual machine-controlled operation.

In mass spectroscopy, the ratios between the strengths of the lower and upper mass constituents of these braces of extremums provide an accurate step of the comparative copiousness of the peptides ( and therefore the proteins ) in the original cell pools because the MS strength response to a given peptide is independent of the isotopic composing of the ICAT reagents.


The usage of ICAT reagent -labelled internal criterions, has now become a common and cardinal pattern in quantitative mass spectroscopy. It has been researched to great advantage in a figure of different Fieldss of biochemistry.

E.1. Quantitative designation of Cancer biomarkers [ 9,10 ]

Figure 5. “ Outline of malignant neoplastic disease biomarker find through quantitative proteomics. “ [ 9 ]

Analytic methods that employ isotope coded affinity tickets are really utile and hence popular in the development of high throughput attack to early malignant neoplastic disease sensing in worlds. [ 9 ] The important quantification and designation of malignant neoplastic disease biomarkers utilizing ICAT reagents is a curative mark for malignant neoplastic disease intervention. In this instance, protein samples incorporating cancerous and non-cancerous cells are denatured and reduced to expose the cysteine -SH peptide residues contained. They can so later labelled with the visible radiation or heavy signifiers of isotope coded affinity tickets in vivo utilizing stable isotopic labelling ( SILAC ; ( e.g. , 2H, 13C, 15N, and 18O ) ) or in vitro utilizing isobaric tickets ( iTRAQ ) . This attack allows expressed proteins and peptides in malignant, cancer-derived cells to be compared with non-cancerous cells. [ 8 ] The usage of labeled peptides as internal criterions allows for comparative and/or absolute appraisal and quantification of the copiousness of the differential proteins present. Emerging engineerings such as the usage of protein microarrays are chances soon being researched and developed for future betterments in malignant neoplastic disease biomarker designation. [ 10 ]

E.2. Quantification of antimalarial drugs and their metabolites in biological fluids [ 7 ]

Malaria is a deathly disease responsible for 1000000s of deceases every twelvemonth, in many tropical and developing states. Antimalarial drugs such as chloroquine, Larium and pyrimethamine and their metabolites ; interact with specific dihydrofolate enzymatic sites in plasmodium falciparum malaria. Since enzymes are mostly made up of proteins, many enzymatic maps are made up of peptide – peptide interactions. Isotope coded affinity tagging combined with high public presentation liquid chromatography has been documented by ‘Kalpesh N. P. et Al, 2010 ‘ [ 7 ] to be a dependable method for the selective finding and quantification of these powerful antimalarial drugs in biological fluids. ICAT reagents are really utile in the extraction phase of the antimalarial drug from a biological matrix as they provide high peptide selectivity and specificity, to avoid intervention from multiple antimalarial combination, or endogenous peptides that exist within the matrix. The usage of the ICAT attack has greatly aided research and development into the pharmacokinetics of different antimalarial drugs particularly Chloroquine. [ 7,8 ]

E.3. Quantification of protein look in oxidative-stressed liver cells as a curative mark for the intervention of liver disease [ 12 ]

A major infective event recurrent in several fluctuations of liver diseases in worlds, involves oxidative emphasis of the liver caused by the formation of reactive O species. Hepatocytes usually have mechanisms responsible for the ordinance of oxidative and anti-oxidative molecules within the cell. However, the presence of reactive O species in the liver affects major cellular constituents including cell proteins, and finally, the cell ‘s regulative ability. This leads to metabolic or proliferative liver disease and eventual cell human death. [ 13 ]

Reactive O species ( ROS ) are mostly represented by chondriosomes and cytochrome P450 enzymes in liver cells. The look of certain protein molecules termed as biomarkers within oxidative-stressed liver cells, and their subsequent quantification utilizing ICAT reagents, can enable an early sensing of liver disease. It can besides let for the progressive monitoring of liver harm as a curative mark to the intervention of liver disease. [ 15 ]

E.4. Quantitative analysis of defaulted proteins present in the encephalon as a curative mark for the intervention of encephalon diseases.

The encephalon is a really complex construction, critical to the being of world. However, a batch of the implicit in mechanisms responsible for the normal map and mis-function of the encephalon have non been to the full researched. Research into quantitatively characterizing the human encephalon proteome and utilizing the analysis to understand of import cell signalling mechanisms [ 16 ] , is a really of import country of neuropoteomics ( i.e. proteomic research and development ) .

The big graduated table usage of stable isotope coded affinity tickets in quantitative analysis of complex encephalon matrixes has helped to supply internal criterions for relevant peptides that are chemically similar but isotopically different. These internal criterions can be used to right place of import biomarkers present in the encephalon as in epilepsy [ 17 ] ; or absent biomarkers as in the pathogenesis of Parkinson ‘s disease [ 18 ] .

E.5. Applications in the proteomic analysis of recombinant proteins

High-throughput attacks to the quantification and designation of proteins, is widely applied in the industrial synthesis of curative enzymes. [ 19 ] Proteomic analysis on most recombinant proteins, battle with really low outputs and hapless solubility which greatly affects the ability to accomplish high-throughput protein purification. Quantitative methods that employ isotope coded affinity tickets have been documented to be the lone manner to accomplish selective high-throughput protein purification with improved outputs, solubility and folding of the recombinant protein, during the procedure [ 19 ] . This is because, purification procedures by biotin affinity normal ensuing in great outputs of over 90 % , doing it really economically favorable. Combinations of two or more isotopic tickets are typically needed to do the most of high-throughput showing. [ 1 ]


The chief application country of isotope coded affinity attack is in the designation of biomarkers as a curative mark for disease intervention and bar. The hereafter of analytical techniques that use Isotope coded affinity tickets for peptide-labelling includes: