New strategies for characterizing ancient proteins using matrix-assisted laser desorption ionization mass spectrometry

Abstract

Structural characterization of ancient proteins is confounded by the small quantity of material remaining in fossils, difficulties in purification, and the inability to obtain sequence information by classical Edman degradation. We present a microbore reversed phase high performance liquid chromatography (rpHPLC) method for partial purification of small quantities (picomoles) of the bone protein osteocalcin (OC) and subsequent characterization of this material by matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The presence of OC in the modern and ancient samples was suggested by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and radioimmunoassay (RIA). The SDS-PAGE of material isolated from 800 yr BP and 10,000 yr BP bones demonstrates a band consistent with the molecular weight of OC and the RIA indicated OC in concentrations of 0.2 to 450ng/mg of bone for samples between 800 and 53,000 yr BP. In modern samples, we demonstrate the use of MALDI-MS to confirm the molecular weight of intact OC and to sequence OC via peptide mass mapping and a novel derivatization approach with post-source decay analysis. MALDI-MS data for three ancient samples with RIA-confirmed osteocalcin (800 yr BP, 10,000 yr BP and 53,000 yr BP) indicate peaks with a molecular mass within the range of modern OC.