Toxicological Sciences, Vol 50, 20-29, Copyright © 1999 by Society of Toxicology
BD Foy, C Toxopeus and JM Frazier
Due to strong binding between organic anions and albumin, the kinetics of
the binding process must be carefully considered in biologically- based
models used for predictive toxicology applications. Specifically, the slow
dissociation rate of an organic anion from the protein may lead to reduced
availability of free anion in its flow through the capillaries of an organ.
In this work, the effect of the dissociation rate of the anion
bromosulphophthalein (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 microM. The uptake of BSP from
the perfusion medium was modeled using a biologically-based kinetic model
of the sinusoidal and intracellular liver compartments. The best fit of the
model to data resulted in the prediction of a slow 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
fit. These results indicate that the strong binding interaction between BSP
and albumin, beyond keeping the concentration of free chemical low due to a
small equilibrium dissociation constant, can also reduce uptake by an organ
due to the slow release of BSP from 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.
ARTICLES
Kinetic modeling of slow dissociation of bromosulphophthalein from albumin in perfused rat liver: toxicological implications
Department of Physics, Wright State University, Dayton, Ohio 45435, USA. bfoy@discover.wright.edu
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