Can we accurately quantify nanoparticle associated proteins when constructing high-affinity MRI molecular imaging probes?

Targeted magnetic resonance contrast agents (e.g. iron oxide nanoparticles) have the potential to become highly selective imaging tools. In this context, quantification of the coupled amount of protein is essential for the design of antibody‐ or antibody fragment‐conjugated nanoparticles. Nevertheless, the presence of magnetic iron oxide nanoparticles is still an unsolved problem for this task. The aim of the present work was to clarify whether proteins can be reliably quantified directly in the presence of magnetic iron oxide nanoparticles without the use of fluorescence or radioactivity. Protein quantification via Bradford was not influenced by the presence of magnetic iron oxide nanoparticles (0–17.2 mmol Fe l−1). Instead, bicinchoninic acid based assay was, indeed, distinctly affected by the presence of nanoparticle‐iron in suspension (0.1–17.2 mmol Fe l−1), although the influence was linear. This observation allowed for adequate mathematical corrections with known iron content of a given nanoparticle. The applicability of our approach was demonstrated by the determination of bovine serum albumin (BSA) content coupled to dextrane‐coated magnetic nanoparticles, which was found with the QuantiPro Bicinchoninic acid assay to be of 1.5 ± 0.2 µg BSA per 1 mg nanoparticle. Both Bradford and bicinchoninic acid assay protein assays allow for direct quantification of proteins in the presence of iron oxide containing magnetic nanoparticles, without the need for the introduction of radioactivity or fluorescence modules. Thus in future it should be possible to make more precise estimations about the coupled protein amount in high‐affinity targeted MRI probes for the identification of specific molecules in living organisms, an aspect which is lacking in corresponding works published so far. Additionally, the present protein coupling procedures can be drastically improved by our proposed protein quantification method. Copyright © 2011 John Wiley & Sons, Ltd. Direct and reliable quantification of proteins in presence of magnetic nanoparticles containing iron oxide (e.g. targeted high‐affinity probes for MRI) via Bradford and BCA protein assays are investigated and established.