ESI-MS can only be accepted as a quantification method when using standards with a high resemblance to the analyte(s). Unfortunately, this is usually not applicable to metallothioneins (MTs), a superfamily of singular metal-binding cysteine-rich proteins, present in all living organisms, since the absence of suitable reference material due to the high diversity among metal-MT species precludes their quantification by molecular mass spectrometry. Even thus, it is widely assumed that the intensities of the ESI-MS peaks of similar species are directly correlated with their relative concentration in the sample, and this has been extended to the determination of different MT proteins coexisting in a sample. Practically all organisms contain several MT isoforms, some of them exhibiting highly similar sequences, with conserved coordinating Cys residues. For the current analysis, we used as a model system the MT isoforms of two terrestrial snails (Helix pomatia and Cornu aspersum). Hence, distinct samples were prepared by mixing, at different molar ratios, the recombinant HpCuMT and HpCdMT isoforms from H. pomatia, or the recombinant CaCuMT, CaCdMT and CaCdCuMT isoforms from C. aspersum, and they were analyzed by ESI-MS both at neutral pH (for Zn-loaded MT forms) and at acidic pH (for the corresponding apo-forms). The results here presented reveal that the ESI-MS peak intensity of a single MT species strongly depends on its sensitivity to be ionized, and thus, on the presence or absence of metal ions bound. Furthermore, our data demonstrate that very similar MT isoforms of the same organism with similar pI (ranging from 7.9 to 8.3) can show a clear different sensitivity to ES ionization, something that cannot be readily predicted only by consideration of their amino acid content. In conclusion, even in this optimum case, deductions about quantity features of MT samples drawn from ESI-MS measurements should be carefully considered.
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