A novel instrument for real time analysis of individual biological cells or other microparticles is described. The instrument is based on inductively coupled plasma time-of-flight mass spectrometry and comprises a three-aperture plasma-vacuum interface, a dc quadrupole turning optics for decoupling ions from neutral components, an rf quadrupole ion guide discriminating against low-mass dominant plasma ions, a point-to-parallel focusing dc quadrupole doublet, an orthogonal acceleration reflectron analyzer, a discrete dynode fast ion detector, and an 8-bit 1 GHz digitizer. A high spectrum generation frequency of 76.8 kHz provides capability for collecting multiple spectra from each particle-induced transient ion cloud, typically of 200-300 micros duration. It is shown that the transients can be resolved and characterized individually at a peak frequency of 1100 particles per second. Design considerations and optimization data are presented. The figures of merit of the instrument are measured under standard inductively coupled plasma (ICP) operating conditions (<3% cerium oxide ratio). At mass resolution (full width at half-maximum) M/DeltaM > 900 for m/z = 159, the sensitivity with a standard sample introduction system of >1.4 x 10(8) ion counts per second per mg L(-1) of Tb and an abundance sensitivity of (6 x 10(-4))-(1.4 x 10(-3)) (trailing and leading masses, respectively) are shown. The mass range (m/z = 125-215) and abundance sensitivity are sufficient for elemental immunoassay with up to 60 distinct available elemental tags. When <15 elemental tags are used, a higher sensitivity mode at lower resolution (M/DeltaM > 500) can be used, which provides >2.4 x 10(8) cps per mg L(-1) of Tb, at (1.5 x 10(-3))-(5.0 x 10(-3)) abundance sensitivity. The real-time simultaneous detection of multiple isotopes from individual 1.8 microm polystyrene beads labeled with lanthanides is shown. A real time single cell 20 antigen expression assay of model cell lines and leukemia patient samples immuno-labeled with lanthanide-tagged antibodies is presented.

Mapping protein-protein interactions is an invaluable tool for understanding protein function. Here, we report the first large-scale study of protein-protein interactions in human cells using a mass spectrometry-based approach. The study maps protein interactions for 338 bait proteins that were selected based on known or suspected disease and functional associations. Large-scale immunoprecipitation of Flag-tagged versions of these proteins followed by LC-ESI-MS/MS analysis resulted in the identification of 24 540 potential protein interactions. False positives and redundant hits were filtered out using empirical criteria and a calculated interaction confidence score, producing a data set of 6463 interactions between 2235 distinct proteins. This data set was further cross-validated using previously published and predicted human protein interactions. In-depth mining of the data set shows that it represents a valuable source of novel protein-protein interactions with relevance to human diseases. In addition, via our preliminary analysis, we report many novel protein interactions and pathway associations.

To identify a rare (e.g., diseased or foreign) cell in a complex mixture, or to understand the proteomic complexity [1,2] of cells, one needs to be able to measure simultaneously and quantitatively a large number of proteins or other biomarkers that may be present in a complex sample. This is a difficult task and is beyond the reach of current capabilities. To address a problem of this complexity, we have begun to develop a high-sensitivity assay [3][4][5][6] based upon elemental tags that will enable the simultaneous measurement of many proteins in a single sample. The advantage of this approach lies in the large number of available elements and isotopes (potentially greater than 79) found in low abundance in biological systems, which will allow multiple tags to be used simultaneously. Inductively coupled plasma mass spectrometry (ICP-MS) is an ideal technique for detecting and quantifying these tags, as ICP-MS provides excellent resolution between the tag masses and an exceptional dynamic range (nine orders of magnitude). [7] This method allows one to overcome some of the limitations of currently available fluorescent tagging approaches.[8] These limitations arise from the spectral overlap of different dyes and the difficulty in measuring simultaneously targets that differ in abundance by an order of magnitude or more. Other benefits of ICP-MS detection include the high sensitivity, which is comparable to that of radioimmunoassays or chemiluminescent assays, [3] insensitivity of elemental tags to photobleaching and storage time, as well as the stability of the tagged sample so that it can be stored or shipped for analysis. We discuss herein the development of a new class of elemental tags for ICP-MS detection and their use for tagging of antibodies chosen to allow specific recognition of distinguishing cell surface markers. By using this technique it should be possible to achieve detection limits on the order of parts per billion, which will allow the detection of cell surface markers with copy numbers as low as 100.Our experimental design is presented in Figure 1. The assay is based upon the concept of a water-soluble polymer bearing multiple metal-chelating ligands. The polymer contains a terminal maleimide group for coupling to cysteine -SH groups on the Fc portion of an antibody. It is now well established that attaching tags to antibodies through -SH groups (generated by selective reduction of disulfide bonds) is much more likely to preserve antibody activity than, for example, the random covalent attachment of tags to the amino group of lysines. The chelating ligand is chosen to form high-affinity complexes with lanthanide (Ln 3+ ) ions. These elements satisfy our requirement for low natural abundance and a wide selection of elements and isotopes. The use of a metal-chelating polymeric tag allows us to incorporate multiple numbers of a given ion, which leads to an increase in the sensitivity of the method, since the ICP-MS signal increases linearly with the number of atoms of a given element. Another impo...

The ubiquitous glyoxalase system converts toxic alpha-keto aldehydes into their corresponding nontoxic 2-hydroxycarboxylic acids, utilizing glutathione (GSH) as a cofactor. The first enzyme in this system, glyoxalase I (GlxI), catalyzes the isomerization of the hemithioacetal formed nonenzymatically between GSH and cytotoxic alpha-keto aldehydes. To study the Escherichia coli GlxI enzyme, the DNA encoding this protein, gloA, was isolated and incorporated into the plasmid pTTQ18. Nucleotide sequencing of the gloA gene predicted a polypeptide of 135 amino acids and Mr of 14 919. The gloA gene has been overexpressed in E. coli and shown to encode for GlxI. An effective two-step purification protocol was developed, yielding 150-200 mg of homogeneous protein per liter of culture. Electrospray mass spectrometry confirmed the monomeric weight of the purified protein, while gel filtration analysis indicated GlxI to be a homodimer of 30 kDa. Zinc, the natural metal ion found in the Homo sapiens and Saccharomyces cerevisiae GlxI, had no effect on the activity of E. coli GlxI. In contrast, the addition of NiCl2 to the growth medium or to purified E. coli apo-GlxI greatly enhanced the enzymatic activity. Inductively coupled plasma and atomic absorption analyses indicated binding of only one nickel ion per dimeric enzyme, suggesting only one functional active site in this homodimeric enzyme. In addition, the apoprotein regained maximal activity with one molar equivalence of nickel chloride, indicative of tight metal binding. The effects of pH on the kinetics of the nickel-activated enzyme were also studied. This is the first example of a non-zinc activated GlxI whose maximal activation is seen with Ni2+.