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

Abstract
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 (65Cu and 67 Zn) were doped to one-dimensional gels of separated tau protein isoforms after gel electrophoresis. In several protein bands metal ions were detected and 65 Cu/63 Cu and 67 Zn/64 Zn 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 proteinsseparated 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.

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