Organic macromolecules in shells of Arctica islandica: comparison with nacroprismatic bivalve shells

A detailed characterisation of the organic composition of Arctica islandica (Linnaeus, 1767) shells with homogenous microstructure is compared with nacroprismatic shells of Pinctada fucata martensii (Dunker, 1872), Hyriopsis cumingii (Lea, 1852) and Diplodon chilensis patagonicus (d’Orbigny, 1835). Thermogravimetric analysis shows lowest total organic contents of 1.65 wt% for A. islandica shells, while all nacroprismatic shells are higher (3.14–4.13 wt%). Macromolecules extracted from the nacroprismatic shells are dominated by hydrophobic amino acids (~ 54%) in the acid extracts, while EDTA-extracts are moderately rich in aspartate and glutamate (16% in total) and in glycine–alanine (42%). In comparison, A. islandica shells have higher concentrations of proline, glycine and aspartate (ca 40%). Infrared spectroscopy shows some acidic protein bands in A. islandica, which cannot be found in the nacroprismatic shells. Alcian Blue and/or modified silver staining methods reveal many prominent bands. Protein bands at around 10, 14, 17, 21, 26, 31, 40 and 55 kDa are detected in A. islandica shells for the first time, thus may constitute a new set of proteins in mollusc shells. SDS-PAGE exhibits apparent molecular weights from 5 to 63 kDa in nacroprismatic shells. Distinct protein bands at around 17 and 26 kDa in A. islandica shells may correspond to a post-translational modification of proteins; these prominent bands, however, are absent in the nacroprismatic samples. Contrarily, SDS-PAGE of both, homogeneous and nacroprismatic shell microstructures show nonacidic-matrix-proteins.