Bio-Rad has developed a workflow linking surface plasmon resonance to MALDI mass spectrometry that could offer a more sensitive and streamlined alternative to conventional SPR-MS techniques.
The method uses an adaptor that allows researchers to insert the company's ProteOn SPR chips directly into a MALDI ionization chamber, reducing loss of sample between steps and leading to higher sensitivity, Bio-Rad applications development manager Fiona Plows told ProteoMonitor.
Using SPR — which allows for real-time, label-free detection and quantification of molecular interactions — with mass spectrometry could prove useful to a variety of research areas such as antibody characterization and protein interaction studies. Efforts to combine the two technologies have proved tricky, however, due to the difficulty of isolating enough sample from the SPR chip to use for a mass-spec run.
Traditionally, Plows said, SPR-MS workflows have used elution-based methods to obtain samples for mass spec after SPR analysis, but "the sensitivity just hasn't been there."
"This technology addresses that problem," she said. "You place the [SPR] chip directly into the [MALDI] ionization chamber via an adapter, you tune the [mass spec] system to be optimized for high amounts of peptide and protein, and then you image it."
At the American Society for Mass Spectrometry's annual meeting in June, Plows and her Bio-Rad colleagues presented a poster demonstrating how they used the workflow to first detect binding events between the 6E10 anti-beta amyloid antibody and three beta-amyloid peptides, and then identify the specific peptide fragments bound.
At the Human Proteome Organization's annual meeting in September, the company plans to present another poster on the method, this one comparing the specificity of the anti-beta amyloid antibodies Ab62658 and 6E10 for the beta-amyloid peptides 1-16, 1-28, and 1-40. In this work the researchers demonstrated that the two antibodies recognize different amyloid peptide fragments. They also showed that the technique was able to work in a sample of 10 percent human serum.
Plows noted the system was able to detect peptides under conditions where elution-based techniques had failed.
"We haven't yet really drilled down to get things quantitative," she said, "but what we wanted to check was if you did a capture experiment the way you normally do for SPR, can you then see [the captured analytes] at all on the mass spec?" she said. "And the answer turns out to be 'Yes.' Essentially, where you couldn't see it with the elution methods, you can see it with this."
The researchers did the mass-spec work on a Bruker Ultraflextreme instrument. Although they have yet to try the method with any other mass-spec machine, in principle it should be compatible with any vendor's MALDI-MS platforms, Plows said.
"There's a geometry constraint in that the geometry of the [mass spec] source has to be such that you can physically fit the SPR chip in it," she said. "We made sure that the SPR chip fit into an adaptor that fit a standard microscope slide, and that fits right into imaging holders that these [mass spec] companies already sell. So we're pretty comfortable that as long as their source takes a standard microscope slide, and their imaging plate is thick enough, then it should take this."
Commercializing the platform is still some ways off, but according to Plows "at the moment we're just looking at the first principles."
"We are very excited about this proof-of-principle work," Laura Moriarty, a Bio-Rad product manager told ProteoMonitor. "We want to start to generate interest and collaborations where we can find out where this could potentially land in terms of applications people might be interested in."
Whether or not the company might partner with specific mass-spec vendors on the platform remains to be seen, she added, saying that while Bio-Rad has "a lot of experience working with other mass-spec vendors, I think at this point it's still very early in the game."
While Plows said Bio-Rad isn't currently interested in targeting the workflow to any particular research purposes, there are, she noted, several areas where the system might be useful.
"One example would be if you were QCing an antibody batch, you could check the activity of that antibody [via SPR] and then you could check its specificity in the mass spec," she said. Similarly, the method could be used to study protein-protein interactions, with the SPR step allowing researchers to measure the binding activity of a particular protein-ligand pair, and MS enabling them to then identify those ligands.
"There's the potential for a number of applications – for example ligand fishing and species identification – and from our perspective we're just working out the potential of the workflow and throwing it out to our customers and seeing what they can create with it in their hands," Plows said.
Bio-Rad is not alone in pursuing SPR-MS workflows. In 2009, researchers at the University of Evry-Val d'Essonne in Evry, France, published a paper in Analytical Chemistry in which they used MALDI-MS to detect the proteins β-lactoglobulin and ovalbumin directly from the surface of an SPR biochip.
The following year researchers at the University of Southern Denmark published in Methods in Molecular Biology protocols for eluting proteins from a Biacore SPR sensorchip for mass spec analysis.
Last week GE Healthcare and Bio-Rad settled a lawsuit related to the Biocore SPR device, which gave Bio-Rad a non-exclusive license to use GE Healthcare's technologies in its SPR products (GWDN 08/15/11). Bio-Rad declined to say if it plans to use the licensed technology in the SPR-MS workflow that Plows described.
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