Identification of plasma proteins facilitated by enrichment on particulate surfaces: analysis by two-dimensional electrophoresis and N-terminal microsequencing.

Plasma protein adsorption on intravenously injectable drug carriers is regarded as an important factor for the fate of the particles in the body after their administration. Therefore, the plasma protein adsorption patterns on a number of different carrier systems were analyzed in vitro employing two-dimensional electrophoresis (2-DE). The particulate systems presented in this study were polystyrene (PS) model particles, PS nanoparticles surface-modified by adsorption of a surfactant, a commercial fat emulsion, and magnetic iron oxide particles used as contrast agents in magnetic resonance imaging. Most of the spots in the plasma protein adsorption patterns could be identified by matching the resulting 2-DE gels with a reference map of human plasma proteins. Several other proteins that indicated preferentially adsorbed proteins on the surface of the particles investigated have either not been identified on the reference map, or their identity was found to be ambiguous. The relevant proteins are all present in plasma in low abundance. Since these proteins were strongly enriched on the surface of the particles, the resulting spots on the 2-DE gels were successfully identified by N-terminal microsequencing. With this approach, two chains of spots, designated PLS:6 and PLS:8, were determined on a plasma reference map: inter-alpha-trypsin inhibitor family heavy chain-related protein (also named PK-120) and a dimer of fibrinogen gamma, respectively. Plasma gelsolin is presented in a 2-DE adsorption pattern of PS model particles. One of the main proteins adsorbed by droplets of a commercial fat emulsion was identified as apoliprotein H. Moreover, the positions of apolipoproteins apoC-II and apoC-III were also verified on the 2-DE protein map of human plasma. Thus, protein adsorption experiments of the kind presented in this study are increasing our insight into human plasma proteins.