Study of formation of Cu- and Zn-containing tau protein using isotopically-enriched tracers by LA-ICP-MS and MALDI-FTICR-MS

Metal-containing proteins were detected directly in separated protein bands in one-dimensional gels, by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) as an atomic mass spectrometric technique. In order to study the binding of Cu and Zn on tau protein isoforms as a target protein in Alzheimer's disease, enriched isotope tracers (Cu-65 and Zn-67) were doped to one-dimensional gels of separated tau protein isoforms after gel electrophoresis. In several protein bands metal ions were detected and Cu-65/Cu-63 and Zn-67/Zn-64 isotope ratios were measured by LA-ICP-MS. The isotope analysis by LA-ICP-MS indicates certain proteins with a natural isotope composition of Cu or Zn. However, copper-containing tau protein isoforms with a changed isotope ratio in comparison to the isotope composition in nature were also found. This experimental finding demonstrates the formation of new metal-containing tau protein complexes during the tracer experiments in the 1-D gel. We attempted to identify the protein bands from tau proteins separated by one-dimensional (1-D) gel electrophoresis by using biopolymer mass spectrometry with MALDI-FTICR-MS, after excision from the 1-D gel and tryptic digestion.