The Gd³⁺ complex of a fatty acid analogue of DOTP binds to multiple albumin sites with variable water relaxivities

The 20 MHz water relaxivity (r₁) of gadolinium(III) complexes formed with two fatty acid analogues of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate) were shown to increase substantially in the presence of albumin. The r₁ values of Gd(C₈-DOTP)^5- and Gd(C₁₁-DOTP)^5- in water were similar to that of the parent GdDOTP^5-, a q = 0 complex known to relax water very efficiently via an outer-sphere mechanism. Neither fatty acid analogue formed apparent aggregates or micelles in water up to 20 mM, but both showed dramatic increases in r₁ upon addition of albumin. Further ultrafiltration studies of Gd(C₁₁-DOTP)^5- in the presence of non-defatted HSA showed that the complex binds at a minimum of five high-affinity fatty acid sites with stepwise binding constants ranging from 1.27 × 10⁵ to 2.7 × 10³ M^-1. The 20 MHz relaxivity of Gd(C₁₁-DOTP)^5- in the presence of excess HSA was 23 mM^-1 s^-1 at 25 °C. The NMRD curve showed a broad maximum 20-30 MHz which fitted well to standard theory for a q = 0 complex with rapid outer-sphere water exchange. The r_1b of Gd(C₁₁-DOTP)^5- bound at the tightest site on HSA was ∼O40 mM^-1 s^-1 at 5 °C, an extraordinarily high value for an outer-sphere complex. However, the r_1b of Gd(C₁₁-DOTP)^5- bound at the weaker sites on HSA was considerably lower, approaching the relaxivity of the free complex in water. This suggests that the complex bound in the highest affinity fatty acid site is less mobile than the same complex bound at the weaker affinity fatty acid sites. This combined ultrafiltration and relaxivity study demonstrates that the common assumption of a single r_1b value for a Gd³⁺ complex bound at several protein sites is not a valid approximation.