These authors contributed equally.
EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels
Article first published online: 4 FEB 2013
Copyright © 2013 John Wiley & Sons, Ltd.
Contrast Media & Molecular Imaging
Volume 8, Issue 3, pages 252–264, May/June 2013
How to Cite
Lagerstedt, J. O., Petrlova, J., Hilt, S., Marek, A., Chung, Y., Sriram, R., Budamagunta, M. S., Desreux, J. F., Thonon, D., Jue, T., Smirnov, A. I. and Voss, J. C. (2013), EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels. Contrast Media Mol Imaging, 8: 252–264. doi: 10.1002/cmmi.1518
- Issue published online: 11 JAN 2013
- Article first published online: 4 FEB 2013
- Manuscript Accepted: 3 NOV 2012
- Manuscript Revised: 30 OCT 2012
- Manuscript Received: 1 MAR 2012
- National Institutes of Health. Grant Numbers: AG029246, T32-GM008799 (NIH-NIGMS)
- US Department of Energy. Grant Number: DE-FG-02-02ER153
- apolipoprotein A-I;
- magnetic resonance imaging (MRI);
- electron paramagnetic resonance (EPR) spectroscopy;
- protein contrast agent;
- high-density lipoprotein (HDL);
- site-directed MRI label
We have engineered apolipoprotein A-I (apoA-I), a major protein constituent of high-density lipoprotein (HDL), to contain DOTA-chelated Gd(III) as an MRI contrast agent for the purpose of imaging reconstituted HDL (rHDL) biodistribution, metabolism and regulation in vivo. This protein contrast agent was obtained by attaching the thiol-reactive Gd[MTS-ADO3A] label at Cys residues replaced at four distinct positions (52, 55, 76 and 80) in apoA-I. MRI of infused mice previously showed that the Gd-labeled apoA-I migrates to both the liver and the kidney, the organs responsible for HDL catabolism; however, the contrast properties of apoA-I are superior when the ADO3A moiety is located at position 55, compared with the protein labeled at positions 52, 76 or 80. It is shown here that continuous wave X-band (9 GHz) electron paramagnetic resonance (EPR) spectroscopy is capable of detecting differences in the Gd(III) signal when comparing the labeled protein in the lipid-free with the rHDL state. Furthermore, the values of NMR relaxivity obtained for labeled variants in both the lipid-free and rHDL states correlate to the product of the X-band Gd(III) spectral width and the collision frequency between a nitroxide spin label and a polar relaxation agent. Consistent with its superior relaxivity measured by NMR, the rHDL-associated apoA-I containing the Gd[MTS-ADO3A] probe attached to position 55 displays favorable dynamic and water accessibility properties as determined by X-band EPR. While room temperature EPR requires >1 m m Gd(III)-labeled and only >10 µ m nitroxide-labeled protein to resolve the spectrum, the volume requirement is exceptionally low (~5 µl). Thus, X-band EPR provides a practical assessment for the suitability of imaging candidates containing the site-directed ADO3A contrast probe. Copyright © 2013 John Wiley & Sons, Ltd.