Comprehensive investigation of the non-covalent binding of MRI contrast agents with human serum albumin

Abstract

Three techniques, electrospray mass spectrometry, ultrafiltration, and proton relaxometry, are compared in the context of the quantitative analysis of non-covalent binding between human serum albumin (HSA) and MRI contrast agents. The study of the affinity by proton relaxometry reveals the association constant and the number of interaction sites assuming that all sites are identical and independent. Ultrafiltration was adapted for the study of paramagnetic complexes. This technique confirmed the results obtained by relaxometry. Electrospray mass spectrometry, an original method able to study non-covalent binding because of its soft ionization process that allows for the survival of weak binding, provides qualitative and quantitative results. Electrospray mass spectrometry confirmed the affinity measured by proton relaxometry and ultrafiltration. This technique requires very small amounts of products and directly gives the stoichiometry of the association, information not easily obtained by classic techniques. Nevertheless, proton relaxometry remains a useful and mandatory technique for determining the enhancement of the relaxation subsequent to the binding although it demands large amounts of compounds. It is to be pointed out that even if the three techniques lead to a similar ranking of the affinity of the contrast agents for HSA, the absolute values of the association constants disagree as a result of the difference in the experimental conditions (presence of salt, native protein or desalted one, approximations in the fitting of the data, liquid or gas phases).

The Protein and Contrast Agent–Specific Influence of Pathological Plasma-Protein Concentration Levels on Contrast-Enhanced Magnetic Resonance Imaging

“…Protein-contrast agent interactions have been modeled with more sophisticated equations before, for example, in the studies of Caravan 8 or Henrotte. 15 However, those require information or assumptions on the contrast-agent protein interaction, for example, the number of independent interaction sites, and could not reproduce the obvious influence on the transverse preYcontrast relaxation rates that albumin and, in particular, IgG exerted in our study. Equation [2] still assumes the relaxation enhancement to linearly grow with contrastagent concentration.…”

Classification and basic properties of contrast agents for magnetic resonance imaging

“…In addition to the liver, it can be used for MRI of the brain and spine. Both Gd 3þ agents significantly bind to HSA, although with affinities at least one order of magnitude lower that for the HSA-binding blood pool agents (65). Because of the resulting increased relaxivity and half-life in blood, they can be also used in MRA applications (66).…”

T1 Relaxivities of Gadolinium-Based Magnetic Resonance Contrast Agents in Human Whole Blood at 1.5, 3, and 7 T

“…Different serum total protein and albumin in each of experimental material setups could be the reason. 20,21 Another reason for discrepancies can be attributed to inaccuracies of concentration determination and /or species-dependent influences of the blood plasma samples. One potential limitation of the present study is that blood oxygenation may influence blood T1 values.…”