QPNC-PAGE
QPNC-PAGE, or quantitative preparative native continuous polyacrylamide gel electrophoresis ("quantitative native PAGE"), is a high-resolution and a highly accurate technique applied in biochemistry and bioinorganic chemistry to separate proteins by isoelectric point. This standardized variant of native gel electrophoresis is used by biologists to isolate active or native metalloproteins in biological samples and to resolve properly and improperly folded metal cofactor-containing proteins or protein isoforms in complex protein mixtures.[1] The high reproducibility and the high-yield electroelution (> 95%) of proteins performed by this technique strongly correlates with the polymerization time (69 hr) of the acrylamide (AA) gels.
As omics platform for quantitative biomedical approaches QPNC-PAGE contributes to the development of metal-based drugs for protein-misfolding diseases,[2] and as such, to the emerging field of biobased economy. Another important link addresses the investigation of the role of environmental contaminants like copper in the etiology of Alzheimer's disease (AD)[3] because inorganic copper[4] that cannot be detoxified completely by the liver, may be a major triggering agent in AD.[5]
Introduction
Proteins perform several functions in living organisms, including catalytic reactions and transport of molecules or ions within the cells, the organs or the whole body. The understanding of the processes in organisms, which are driven by chemical reactions, depends to a great extent on our ability to isolate active proteins in biological samples for more detailed examination of chemical structure and physiological function. As about 30-40% of all known proteins contain one or more metal ion cofactors (e.g., ceruloplasmin), especially native metalloproteins have to be isolated, identified and quantified in biomatrices. Many of these cofactors play a key role in vital enzymatic catalytic processes or stabilize globular protein molecules.[6] Therefore, the electrophoresis and other separation techniques are highly relevant as initial step of protein analysis followed by mass spectrometric and magnetic resonance methods for quantifying and identifying the proteins of interest.
In this context, QPNC-PAGE is introduced as part of a combined procedure for biomedical approaches to investigate protein-protein interactions and protein-cofactor interactions in clinical samples. A major goal of this article is to elucidate the relationship between the polymerization time of acrylamide gels, gel stability and analytical results obtained by the following procedure.
Method
Separation and buffering mechanisms

In gel electrophoresis proteins are normally separated by charge and size. The aim of isoelectric focusing (IEF) is to separate proteins according to their isoelectric point (pI), thus, according to their charge at different pH values.[7] Here, the same mechanism is accomplished in a commercially available electrophoresis chamber (see figure Equipment) for separating charged biomolecules, for example, superoxide dismutase (SOD)[8] or allergens,[9] at continuous pH conditions and different velocities of migration depending on different isoelectric points.[10]
Due to the specific properties of the prepared gel and electrophoresis buffer solution (which is basic and contains Tris-HCl and NaN3), most proteins of a biological system are charged negatively in the solution, and will migrate from the cathode to the anode due to the electric field. At the anode, electrochemically-generated hydrogen ions react with Tris molecules to form monovalent Tris ions. The positively charged Tris ions migrate through the gel to the cathode where they neutralise hydroxide ions to form Tris molecules and water. Thus, the Tris-based buffering mechanism causes a constant pH in the buffer system.[11]
Although the pH value (10.00) of the electrophoresis buffer does not correspond to a physiological pH value within a cell or tissue type, the separated ring-shaped protein bands are eluted continuously into a physiological buffer solution (pH 8.00) and isolated in different fractions (see figure Electropherogram). Provided that irreversible denaturation cannot be demonstrated (by an independent procedure), most protein molecules are stable in aqueous solution, at pH values from 3 to 10 if the temperature is below 50 °C.[12] As the Joule heat generated during electrophoresis may have a negative impact on the stability and migration behavior of proteins, the separation system, including the electrophoresis chamber and a fraction collector, is cooled in a refrigerator at 4 °C (see figure Equipment).
Gel properties and polymerization time
Best polymerization conditions are obtained at 25-30 °C[13] and polymerization seems terminated after 20-30 min of reaction although residual monomers (10-30%) are detected after this time.[14] The time of polymerization of the gel may directly affect the peak-elution times of separated metalloproteins in the electropherogram due to the compression of the gels and their pores with the longer incubation times (see figure Electropherogram). In order to insure maximum reproducibility in gel pore size and to obtain a fully polymerized and non-restrictive large pore gel for a PAGE run, the polyacrylamide gel is polymerized for a time period of 69 hr at room temperature (RT). After this time the gel is in its lowest energy state and ready to be run. The exothermic heat generated by the polymerization processes is dissipated constantly. As a result, the prepared gel is homogeneous, inherently stable and free of monomers or radicals. Fresh polyacrylamide gels are further hydrophilic, electrically neutral and do not bind proteins.[15]
The 4% T, 2.67% C gel is pre-run to equilibrate it. It is essentially non-sieving and optimal for electrophoresis of proteins that are smaller and larger than 200 ku (cf. agarose gel electrophoresis). Proteins migrate in it more or less on the basis of their free mobility.[16] For these reasons interactions of the gel with the biomolecules are negligibly low and the proteins separate cleanly and predictably (see figure Electropherogram). The separated metalloproteins (e.g., metal chaperones, prions, metal transport proteins, amyloids, metalloenzymes, metallopeptides, metallothionein) are not dissociated into apoproteins and metal cofactors.[17]
Reproducibility and recovery

The bioactive structures (native or 3D conformation or shape) of the isolated protein molecules do not undergo any significant conformational changes. Thus, active metal cofactor-containing proteins can be isolated reproducibly in the same fractions after a PAGE run (see figure Electropherogram). A shifting peak in the respective electropherogram may either indicate that a denatured metalloprotein is available in a complex protein mixture to be separated or the standardized time of gel polymerization (69 hr, RT) is not implemented in a PAGE experiment. A lower deviation of this standardized polymerization time (< 69 hr) stands for incomplete polymerization, whereas exceeding this time limit (> 69 hr) is an indicator of gel aging (see figure Electropherogram). Under standard conditions metalloproteins with different molecular mass ranges and isoelectric points have been recovered in biologically active form at a quantitative yield of more than 95%.[18]
By preparative SDS polyacrylamide gel electrophoresis standard proteins (cytochrome c, aldolase, ovalbumin and bovine serum albumin) with molecular masses of 14–66 ku can be recovered with an average yield of about 73,6%.[19] Preparative isotachophoresis (ITP) is applied for isolating palladium-containing proteins with molecular masses of 362 ku (recovery: 67%) and 158 ku (recovery: 97%).[20]
Quantification and identification
Low concentrations (ppb-range) of Fe, Cu, Zn, Ni, Mo, Pd, Co, Mn, Pt, Cr, Cd and other metal cofactors can be identified and absolutely quantified in an aliquot of a fraction by inductively coupled plasma mass spectrometry (ICP-MS)[21] or total reflection X-ray fluorescence (TXRF),[22] for example. In case of ICP-MS the structural information of the associated metallobiomolecules[23] is irreversibly lost due to ionization of the sample with plasma.[24] Because of high purity and optimized concentration of the separated metalloproteins, for example, therapeutic recombinant plant-made pharmaceuticals such as copper chaperone for superoxide dismutase (CCS) from medicinal plants, in a few specific PAGE fractions, the related structures of these analytes can be elucidated quantitatively by using solution NMR spectroscopy under non-denaturing conditions.[25]
Applications
Improperly folded metal proteins, for example, CCS or Cu/Zn-superoxide dismutase (SOD1) present in brain, blood or other clinical samples, are indicative of neurodegenerative diseases like Alzheimer's disease or Amyotrophic Lateral Sclerosis (ALS).[26] Active CCS or SOD molecules contribute to intracellular homeostatic control of essential metal ions (e.g., Cu1+/2+, Zn2+, Fe2+/3+, Mn2+, Ni3+) in organisms, and thus, these biomolecules can balance pro-oxidative and antioxidative processes in the cytoplasm. Otherwise, transition metal ions take part in Fenton-like reactions in which deleterious hydroxyl radical is formed, which unrestrained would be destructive to proteins.[27] The loss of (active) CCS increases the amyloid-β production in neurons which, in turn, is a major pathological hallmark of AD.[28] Therefore, copper chaperone for superoxide dismutase is proposed to be one of the most promising biomarkers of Cu toxicity in these diseases.[29] CCS should be analysed primarily in blood because a meta-analysis of serum data showed that AD patients have higher levels of serum Cu than healthy controls.[30]
Quantitative native PAGE is applied in the field of molecular biology to purify enzymes and recombinant proteins of microbial strains.[31] The thermostability and activity of enzymes expressed by thermophilic bacteria is genetically encoded. In the frame of bioeconomy these biomolecules can be used as research reagents and as catalysts for industrial processes.[32]
Principle
History
In the 20th century it was generally accepted that ammonium persulfate (APS)/TEMED-initiated reactions should be allowed to proceed for 5-15 min[33] to approx. 1-2 hr[34] to ensure maximum reproducibility in gel pore size of PAGE gels. In another review it is recommended to allow the gel to polymerize overnight at room temperature.[35] Longer incubation times (16 hr to 2 wk) to finish the matrix formation might not have any essential effects on the protein separation process.[36] Explicit polymerization times for specific applications or methods are not mentioned in the literature.[37] Being hydrolyzed into polyacrylic acid polyacrylamide gels were further considered as inherently unstable with respect to polymer resistance to alkaline media.[38] Under these conditions only a semiquantitative characterization of metalloproteins was enabled.[39] Therefore, the relationship between the polymerization time, gel stability and analytical results had to be investigated addressing the absolute quantification of specific protein molecules:
In 2001 a new basic principle of gel electrophoresis was discovered at the Forschungszentrum Jülich, applied for patent in 2003 and subsequently issued in 2014.[40] This invention provided the first conclusive evidence that the time of polymerization of a polyacrylamide gel directly affects the result of protein purification because the separation properties implying the mechanical and chemical stability of a gel and its pores are determined by this parameter. As a consequence, the peak-elution times of the separated metalloproteins in the electropherogram may vary considerably (see figure Electropherogram). On the other hand, the results of protein electrophoresis can be optimized in terms of reliability and avoiding artifacts by strict adherence to a standardized time of polymerization (69 hr, RT) for acrylamide gels implying inherent stability at basic pH. This optimization process of selected electrophoretic parameters paved the way to "quantitative native PAGE" by the Jülicher researcher Bernd Kastenholz.[41]
Although the quality, quantity and mixing ratio of TEMED, APS and AA/Bis-AA as well as the ambient temperature are the most important factors to initiate and impel the polymerization reaction of PAGE gels, it is evident that the time of polymerization is the limiting factor in these chemical processes. Systematic investigations of the gel stability over time reveal significant changes in gel structure by day 3 (72 hr) after polymerization (5-15 min).[42] These results are in excellent agreement with the finding that the gel aging begins after 69 hr gelation time, as reported here. Given these new findings a re-assessment of past results and published data related to protein electrophoresis[43] and quantitative proteome analysis is necessary.[44]
Persons
First publications (2009) concerning the medical applications of this technique were edited or co-authored by the Münsteran human geneticist Prof. Jürgen Horst and the well-known American scientist and internationally recognized expert in the fields of DNA sequence analysis and protein electrophoresis David E. Garfin. In the 1970s and the early 1980s Dr. Garfin became one of the most important investigators and early pioneers in the field of prion research (scrapie) in the team of Stanley B. Prusiner at the UC SF.[45] In addition to his pioneering work on one- and two-dimensional gel electrophoresis in the 1990s at Bio-Rad Laboratories[46] he became co-editor of the Handbook of Isoelectric Focusing and Proteomics (2005)[47] and co-authored a worth reading chapter on bioseparation methods in the Kirk-Othmer Encyclopedia of Chemical Technology (2007).[48] For significant contributions to electrophoresis in both engineering and biology communities Dave Garfin received the 2013 AES Electrophoresis Society Career Award in San Francisco.[49]
Biomedicine
Said authors anticipated that the above-mentioned approach might be implemented in the therapy and diagnosis of several protein-misfolding diseases: on the one hand, copper chaperone for superoxide dismutase may serve as a biomarker for Cu toxicity in neurodegenerative diseases, on the other hand, copper chaperones[50] and other non-proteinogenic metallopharmaceuticals (e.g., Cu orotate,[51] Li chloride[52]) are indicated as lead compounds for the etiological treatment of Alzheimer's disease.[53]
As the mis-localization of metal ions (in particular Cu[54]) in the cell is most likely responsible for the onset and progression of sporadic and genetic forms of Alzheimer's disease and other dementias, said compounds may pass the blood brain barrier and trigger a metal-mediated signaling cascade of biochemical reactions that restore and maintain metal homeostasis in order to preserve the neuronal function in the brain of AD patients.[55] As cellular responses to these reactions the production of amyloid-β peptides and oxidative processes are normalized and neuritic plaques of AD brains are degraded by upregulation of the proteasome and other molecular mechanisms.[56] In these processes, especially protein-protein interactions play a crucial role. In general, the relative biochemical impact of an applied metal-containing compound (metal-based drug) is depending on its dose, bioavailability, trace metal binding form (chemical form) and accuracy of quantitative measurement.[57]
According to the Hofmeister series salt effects could be another approach for the treatment of Alzheimer's disease by using certain salts for dissolving protein aggregates or inhibiting amyloid formation.[58] Adding salts to complex protein mixtures, however, may induce a shift of isoelectric points, and thus, affect protein structure and activity. In this context, one study found that kosmotropic salts (e.g., ammonium sulfate) that were used for the precipitation of native proteins in solution, subsequently may result in denatured high molecular weight metalloproteins in the same solution.[59] It can be concluded that ions which promote aggregation, simultaneously cause denaturation of the native conformation. In opposite case chaotropic salts force globular proteins to unfold.[60] For these reasons, restoring and maintaining the physiological states of aggregated or unfolded (metal) proteins by applying Hofmeister salts in the therapy of Alzheimer's disease is very unlikely to happen. For example, Li chloride caused a reduction in protein synthesis and hence the level of amyloid-β peptides, however, this compound may also generate severe side effects induced by long-term, high-dose lithium.[61]
Curcumin is one of the most promising therapeutic agents for inflammation, cystic fibrosis, Alzheimer's disease and cancer because these biomolecules scavenge radicals and maintain the levels of (active) antioxidant enzymes (e.g., SOD1)[62] in the presence of copper. Curcumin is almost free from side effects, however, limited for application due to its poor bioavailability.[63] The development and production of plant-made pharmaceuticals (e.g., CCS) by combining modern biochemical techniques and plant phenotyping platforms,[64] may help to provide bioactive therapeutic proteins as a major basis for pharmacological efficiency in conformational diseases. Said molecules are non-toxic, reveal a high specificity and their mechanisms of action are well known in living organisms.[65]
Conclusions
High protein yield and purity are the bottleneck of quantitative native protein analysis in biological samples. QPNC-PAGE is a unique method and the initial step that opens the bottleneck of protein isolation in complex protein mixtures. This preparative technique is based on a new principle and a new constant of acrylamide gel electrophoresis implying the accurate control of gel pore size and stability by the time of polymerization of acrylamide. A combined procedure of solution NMR, QPNC-PAGE, and ICP-MS may be the key for the diagnosis and therapy of several protein-misfolding diseases related to dyshomeostasis of biometal metabolism in the human brain. Biological proteins possess the pharmacological potential to restore and maintain the homeostasis of metal ions in conformational diseases.
See also
References
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External links
- Chapters on 1-D / 2-D Gel Electrophoresis, and Isoelectric Focusing are provided by AES Electrophoresis Society Application Foci
- Standard protocol for "quantitative native PAGE" available on ResearchGate
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