Molecular size of native proteins of Mytilus serum which contains a dominant fraction with heavy metal-binding properties

Abstract

Native serum of Mytilus edulis was investigated by gradient PAGE. Protein composition varied between individual mussels. However, six major bands were obtained relatively constantly in electropherograms revealing molecular weights of 35±1.8 kDa, 136±4.0 kDa, 287±16.9 kDa, 423±26.3 kDa, 498±7.5 kDa and 667±24.8 kDa. The serum protein band 1 (SPB1) of ∼35 kDa occurred in all mussels at a relatively high concentration. From two very different experimental approaches information was expected about a physiological function of this prominent protein fraction: (1) No support was obtained for its postulated vitellogenin activity as it was not found in eggs. (2) On the other hand, SPB1 reacted strongly with heavy metals. It was precipitated by cadmium acetate and bound to organomercurial agarose. Furthermore, evidence was obtained that most of the Cd in serum from contaminated mussels was complexed with humoral components as about 75% of the serum-metal was retained by an ultrafiltration membrane (molecular weight cutoff 3 kDa). In contrast, a seawater solution of cadmium passed the filter pores as determined by atomic absorption chromatography. It is suggested that SPB1 could be involved in heavy metal transport between gills, considered to be the primary sites of metal entry, and other organs accumulating the uptaken water pollutants.