title: Quantitation of Human Metallothionein Isoforms in Cells, Tissues, and Cerebrospinal Fluid by Mass Spectrometry.
abstract: Metallothioneins (MTs) are a family of small, highly conserved, cysteine-rich metal-binding proteins that are important for zinc and copper homeostasis, protection against oxidative stress, and buffering against toxic heavy metals. Individual human MT isoforms are candidate biomarkers for heavy metal toxicity, and selected cancers and neurodegenerative diseases. The similar antigenicity of human MT-1 and MT-2 isoforms precludes development of antibody-based assays for their individual quantitation. Metal-based MT quantitation methods do not directly measure MT isoforms. A bottom-up mass spectrometry-based approach solves these problems by exploiting the unique masses and chromatographic properties of the acetylated N-terminal tryptic peptides of MT isoforms. These unusually hydrophilic 20- to 21-residue peptides contain five invariant cysteines. Strong cation exchange chromatography separates them from bulk internal tryptic peptides. Reversed-phase chromatography further separates them from more hydrophobic peptides of similar mass. Absolute quantitation is obtained by adding MT peptide standards alkylated with15N-iodoacetamide to biological samples alkylated with14N-iodoacetamide. Accurate quantitation is enhanced by dimethyl sulfide treatment to reverse oxidation of the N-terminal methionine. Originally optimized for measuring MT isoforms in cell lines, the method has been adapted to quantify MT isoforms in brain tissue and cerebrospinal fluid. The method can also be adapted for relative quantitation of MT isoforms between matched biological samples. It cannot be used to measure human MT-4 because of an arginine at position 4. Except for this type of limitation, the method is applicable to MT quantitation in many other species.
authors.affiliation: University of North Dakota School of Medicine and Health Sciences, University of North Dakota, Grand Forks, ND, United States. Electronic address: john.shabb@med.und.edu.
authors.forename: J B
authors.initials: JB
authors.lastname: Shabb
authors.affiliation: University of North Dakota School of Medicine and Health Sciences, University of North Dakota, Grand Forks, ND, United States.
authors.forename: W W
authors.initials: WW
authors.lastname: Muhonen
authors.affiliation: University of North Dakota School of Medicine and Health Sciences, University of North Dakota, Grand Forks, ND, United States.
authors.forename: A A
authors.initials: AA
authors.lastname: Mehus
mesh_terms: D000595:Amino Acid Sequence; D001921:Brain; D002460:Cell Line; D002851:Chromatography, High Pressure Liquid / Q000592:standards; D002852:Chromatography, Ion Exchange; D006801:Humans; D008668:Metallothionein / Q000134:cerebrospinal fluid / Q000737:chemistry / Q000302:isolation & purification*; D020033:Protein Isoforms / Q000134:cerebrospinal fluid / Q000737:chemistry / Q000302:isolation & purification; D012015:Reference Standards; D020539:Sequence Analysis, Protein; D053719:Tandem Mass Spectrometry / Q000592:standards
chemical_list: D020033:Protein Isoforms; D008668:Metallothionein
keywords: (15)N-iodoacetamide; Absolute quantitation; Biomarkers; Cerebrospinal fluid; Isoforms; Mass spectrometry; Metallothionein; Methionine sulfoxide; Targeted proteomics