Site-specific water proton relaxation enhancement of iron(III) chelates noncovalently bound to human serum albumin.

Binding of potential blood pool and hepatobiliary paramagnetic iron(III) contrast agents, rac- and meso-Fe(5-Br-EHPG)- (iron(III) N,N'-ethylenebis [(5-bromo-2-hydroxyphenyl)glycinate]) and Fe(5-Br-HBED)- (iron(III) N,N'-bis-(5-bromo-2-hydroxybenzyl)ethylenediaminediacetic acid) to human serum albumin (HSA) has been studied using the proton relaxation enhancement (PRE) effect on solvent protons. These chelates bind avidly to multiple sites on HSA with binding constants on the order of 10(4) to 10(5) M-1. Interestingly, binding results in a decrease in the diamagnetic component of the water relaxivity due to HSA, while the expected enhancement of the paramagnetic component of water proton relaxation rates occurs due to the increase in the rotational correlation times of the protein-bound agents. These relaxation enhancements are variable, depending upon the site on the protein to which these chelates are bound, and can be as high as approximately 7 mM-1 s-1 at 5 degrees C and approximately 5 mM-1 s-1 at 37 degrees C at 20 MHz (enhancements of approximately 2-5). Change of temperature from 5 to 37 degrees C also appears to switch the relative affinities of these chelates for their primary and secondary binding sites. It is found that the important HSA binding site for the heme breakdown product, bilirubin-IX alpha, is a target for these agents and is the site of highest relaxivity for all the agents.