Kinetic modeling of slow dissociation of bromosulphophthalein from albuminin perfused rat liver: Toxicological implications

Due to strong binding between organic anions and albumin, the kinetics of t he binding process must be carefully considered in biologically-based model s used for predictive toxicology applications. Specifically, the slow disso ciation rate of an organic anion from the protein may lead to reduced avail ability of free anion in its how through the capillaries of an organ, in th is work, the effect of the dissociation rate of the anion bromosulphophthal ein (BSP) from albumin was studied in isolated, perfused rat livers in the presence of albumin concentrations of 0.25, 1, and 4% (w/v) and an initial BSP concentration of 20 mu M. The uptake of BSP from the perfusion medium w as modeled using a biologically-based kinetic model of the sinusoidal and i ntracellular liver compartments. The best fit of the model to data resulted in the prediction of a dow dissociation rate constant for the BSP-albumin of between 0.097 and 0.133 s(-1). Assuming BSP and albumin to be in binding equilibrium in the sinusoidal space, with rapid binding-rate constants, as is often done, produced an unacceptable Bt. These results indicate that th e strong binding interaction between BSP and albumin, beyond keeping the co ncentration of free chemical low due to a small equilibrium dissociation co nstant, can also reduce uptake by an organ due to the slow release of BSP f rom the protein during passage through the capillaries. The implication of this dissociation-limited condition, when extrapolating to other doses and in-vivo situations, is discussed.