A multiway approach for classification and characterization of rabbit liver apothioneins by CE-ESI-MS.

ABSTRACT

We applied a multiway approach to extract information from the analysis of protein isoforms by CE-ESI-MS. Metallothioneins (MT) are low-molecular-weight proteins (6-7 kDa) with a strong affinity for heavy-metal ions. Rabbit liver MT-I and MT-II fractions are purified from MT samples. At low pH, the bound metal ions were released from the amino acid structures, giving rise to apothioneins. MT-I, MT-II and MT apothioneins, which are complex mixtures of protein isoforms, were analyzed by CE-ESI-MS. After data pre-processing, parallel factor analysis (PARAFAC) and multivariate curve resolution-alternating least squares (MCR-ALS) were applied to the data sets. In both cases, the models enabled classification of the protein samples and identification of their characteristic sub-isoforms using a set of three components. MCR-ALS required an initial estimate of the pure mass spectra of the three components. Thus, PARAFAC loadings were used to initialize the MCR-ALS optimization. The classifications obtained with MCR-ALS were slightly better than those obtained with PARAFAC, probably because MCR-ALS was less affected by the small migration time shifts of the pre-processed electropherograms. However, no differences were found between the pure mass spectra of the three components in either model. Finally, MCR-ALS allowed us to obtain an individual electrophoretic profile of each of the three components for each of the samples analyzed, which proved valuable for characterization and quantification purposes.