Physico-chemical characterization of metal binding proteins using HPLC-ICP-MS, HPLC-MA-AAS and electrospray-MS

Quantification of snake venom proteomes by mass spectrometry-considerations and perspectives

The advent of soft ionization mass spectrometry-based proteomics in the 1990s led to the development of a new dimension in biology that conceptually allows for the integral analysis of whole proteomes. This transition from a reductionist to a global-integrative approach is conditioned to the capability of proteomic platforms to generate and analyze complete qualitative and quantitative proteomics data. Paradoxically, the underlying analytical technique, molecular mass spectrometry, is inherently nonquantitative. The turn of the century witnessed the development of analytical strategies to endow proteomics with the ability to quantify proteomes of model organisms in the sense of "an organism for which comprehensive molecular (genomic and/or transcriptomic) resources are available." This essay presents an overview of the strategies and the lights and shadows of the most popular quantification methods highlighting the common misuse of label-free approaches developed for model species' when applied to quantify the individual components of proteomes of nonmodel species (In this essay we use the term "non-model" organisms for species lacking comprehensive molecular (genomic and/or transcriptomic) resources, a circumstance that, as we detail in this review-essay, conditions the quantification of their proteomes.). We also point out the opportunity of combining elemental and molecular mass spectrometry systems into a hybrid instrumental configuration for the parallel identification and absolute quantification of venom proteomes. The successful application of this novel mass spectrometry configuration in snake venomics represents a proof-of-concept for a broader and more routine application of hybrid elemental/molecular mass spectrometry setups in other areas of the proteomics field, such as phosphoproteomics, metallomics, and in general in any biological process where a heteroatom (i.e., any atom other than C, H, O, N) forms integral part of its mechanism.

Characterization of raft mussels according to total trace elements and trace elements bound to metallothionein-like proteins

In the current work, samples of Mytilus galloprovincialis collected in different sites from Ría de Arousa estuary (Galicia, north-western Spain) were analysed for total Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn, and for Ba, Cu, Mn and Zn bound to metallothionein-like proteins isoform I (MLP-I) contents. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to assess total metal contents in raft mussels, while high performance liquid chromatography (HPLC) with an on-line metal detection with ICP-OES was used to measure metals bound to MLP-I. A microwave assisted acid digestion procedure was used as a sample pre-treatment for total metal contents, while a cytosolic preparation method based on a blending procedure with TRIS-HCl (pH 7.4) as an extracting solution was used to isolate MLP-I. Concentrations of total metals and metals bound to MLP-I were used as discriminating factors to establish different regions in the Ría de Arousa estuary. Principal component analysis (PCA) and cluster analysis (CA) were used as unsupervised pattern recognition procedures, and the half-range central value transformation was used as a data pre-treatment to homogenize data sets. Results have revealed a separation of raft mussels in good agreement with water circulation pattern and oceanographic processes in the estuary only when concentrations of metals to bound MLP-I are used as discriminating factors. Otherwise, raft mussels are classified as samples harvested in the inner or outer sides of the Ría.

Characterization of metallothionein isoforms from rabbit liver by liquid chromatography coupled to electrospray mass spectrometry

Metallothioneins (MTs), a group of low molecular weight proteins found in practically all life forms, are characterized by high sulfur content and an affinity for metal ions. At acidic pH, MTs show metal depletion, leading to apothioneins. In the work described here, in order to optimize the separation of rabbit liver apothioneins using liquid chromatography (LC) with UV detection, the proportion of the organic modifier of the mobile phase was optimized by establishing relationships between Reichardt's E(N)(T) scale of solvent polarity and the chromatographic retention measured by the capacity factor, k. Additionally, such optimum separations were carried out in a LC-electrospray mass spectrometry (ES-MS) coupled system allowing the identification and characterization of the different rabbit liver apo-MT-forms. In this way, electrospray ionization mass spectrometry offers great possibilities aiming at a better understanding of metallothionein polymorphism.

Investigation of metal complexes with metallothionein in rat tissues by hyphenated techniques

The potential of hyphenated techniques based on a combination of microbore reversed-phase (RP) HPLC or capillary zone electrophoresis (CZE) with inductively coupled plasma (ICP) or electrospray (ES) mass spectrometry (MS) was demonstrated for the characterization of metal complexes with metallothionein in rat liver and kidney. The mixture of MT complexes was isolated from the tissues by size-exclusion LC and further characterized in neutral pH conditions (pH 6.8-7.2) by RP-HPLC or CZE. The metal stoichiometry and the molar mass of the eluted complexes was measured by ICP-MS and ES-MS, respectively. An additional dimension to the analysis was achieved by post-column acidification of the chromatographic eluent that allowed the determination of the molecular weight of the demetallated complexes with 10-fold higher sensitivity. The approach allowed the detection of two major metallothionein (MT) isoforms (MT-1 and MT-2) in liver and one MT isoform in kidney. The actual number of peaks in chromatograms and electropherograms was bigger because of the formation of mixed Cd-Cu complexes of the same MT isoform that showed different hydrophobicities.

Recent developments in quantification methods for metallothionein

The metallothioneins (MT), a family of proteins with relatively low molecular weight (6-7 kDa), are characterised by the intrinsic presence of 20 cysteinyl groups in their structure, which confers unique metal binding properties to the molecule. Since MT are involved in biological roles, quantification of MT remains an important task. To date, a large number of determination methods have been developed. In this paper recent developments, from 1995 to the present, in methodology employed in quantification studies of total MT and MT polymorphism are described. Different fields were taken into consideration, such as (i) separation techniques and hyphenated systems, (ii) electrochemical methods, (iii) immunological methods and (iv) quantification of MT mRNA. The data presented are based on our own and published results. A brief overview of the use of metallothionein as a biomarker is included as a relevant example of the importance of MT quantification. Finally, general problems associated with determination and evaluation of obtained results within the above four topics are mentioned.

Characterization of metallothionein isoforms by reversed-phase high-performance liquid chromatography with on-line post-column acidification and electrospray mass spectrometric detection

Post-column acidification of the chromatographic effluent was developed to eliminate artefacts in investigations of the polymorphism of metallothionein (MT) by microbore reversed-phase HPLC with detection by pneumatically assisted electrospray mass spectrometry. Metallated species (Cd, Zn and mixed Cd-Zn complexes) were decomposed on-line to produce apo metallothioneins of which the molecular masses were determined by MS. Besides the simplification of the mass spectra taken at the apexes of the chromatographic peaks, the method resulted in a 10-fold improvement of the detection limit of metallothionein and allowed a more comprehensive and less ambiguous detection and identification of the iso- and subisoforms. The method was applied to the characterization of rabbit liver metallothioneins: rabbit liver MT (purified by size-exclusion chromatography only) and MT-1 and MT-2 isoform fractions purified additionally by anion-exchange chromatography.

Inhibition, reactivation, and determination of metal ions in membrane metalloproteases of bacterial origin using high-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry

High-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) was used for the characterization of metal ions in several metalloproteases of bacterial origin. The different components of the bacterial extracts were separated on a size-exclusion column. The eluent of the HPLC system was continuously transported to the ICP-MS system for rapid, reproducible, and sensitive analyses of trace elements in the metalloproteases. Two different membrane proteases from Bacillus cereus and Pseudomonas aeruginosa were characterized to be zinc metalloproteases using enzymological methods and HPLC-ICP-MS. The zinc content was determined to be three molecules of zinc per protein molecule for the B. cereus protease and one molecule of zinc per protein molecule for the P. aeruginosa protease. For another purified protease, a periplasmic alanyl aminopeptidase of P. aeruginosa, the lack of protein-bound metal ions could be clearly determined-a confirmation that this main aminopeptidase of P. aeruginosa belongs to the cysteine protease family. The presence of nonionic detergents can influence the distribution of trace elements during the HPLC separation. Therefore, the use of these substances should be avoided during enzyme purification for metal analyses or they should be exchanged later for zwitterionic and ionic detergents with more strongly dissociating properties.