Albumin-mediated transport of rose bengal by perfused rat liver. Kinetics of the reaction at the cell surface.

Abstract

A B S T R A C T Rapid dissociation of organic anions from plasma albumin maximizes the presentation of free ligand to the cell surface and thus favors its efficient hepatic extraction. Even assuming these optimal conditions, however, taurocholate and rose bengal have hepatic extraction fractions that are higher than can be accounted for by spontaneous dissociation of their albumin-ligand complexes. In this study we developed a transport model that attributes this behavior to sites on the hepatocyte plasma membrane that bind the albumin-ligand complexes, promoting the transport of ligand into the hepatocyte. Fitting this model to rose bengal removal rates measured over a wide range of albumin concentrations yields estimates of the number of cell surface sites and their affinity for albumin. These estimates are in good agreement with those reported by Weisiger, Gollan, and Ockner for the binding of ligand-free albumin to isolated hepatocytes. We conclude that both experiments measure the same phenomenon and, accordingly, that the binding of albumin to the cell surface is the functional equivalent of albumin-mediated transport.

Plasma binding and transport of diazepam across the blood-brain barrier. No evidence for in vivo enhanced dissociation.

“…Importantly, the transport of unbound ligand under nonequilibrium conditions appears theoretically to be facilitated as a function of the albumin concentration and in such a way that the uptake of total ligand is similar to that which would occur if enhanced dissociation was present (31). Probably this nonequilibrium effect also accounts for the apparent protein-mediated uptake ofother compounds by the brain (7,11,(13)(14)(15) and liver (6-9) using the single-pass, rapid injection technique, and may be a possible factor with other experimental approaches (1)(2)(3)(4)(5)10).…”

“…However, the validity of this free-drug (hormone) hypothesis has been challenged by recent experimental observations on the transport of certain highly bound ligands. For example, the hepatic uptake of a number of organic anions (1)(2)(3)(4)(5), steroids (6,7), thyroxine (8,9), and drugs (10) is greater than predicted by their in vitro binding characteristics, and the rate of transport correlates better with the albumin-bound concentration than the unbound level. The brain extraction of drugs such as valproic acid (1 1) and benzodiazepines (12) as well as steroids (7,(13)(14)(15) and thyroid hormones (7) has also been interpreted to indicate a role for albumin in transmembrane transport beyond that associated with the simple binding interaction.…”

At physiologic albumin/oleate concentrations oleate uptake by isolated hepatocytes, cardiac myocytes, and adipocytes is a saturable function of the unbound oleate concentration. Uptake kinetics are consistent with the conventional theory.

“…In 1981, virtually simultaneously, Weisiger et al and Forker and Luxon highlighted other apparent pitfalls of the traditional theory ( 13,14). They reported, inter alia, that hepatic uptake velocity for several ligands (a) exceeded the reported in vitro dissociation rates of the corresponding albumin/ligand complexes; (b) appeared to be a function of total rather than unbound ligand; (c) was apparently competitively inhibited by the addition of uncomplexed albumin; and (d) appeared to saturate with increasing albumin rather than ligand concentrations (13)(14)(15)(16)(17)(18). These authors interpreted their kinetic data as indicating some form of facilitated dissociation of the albumin ligand complexes, mediated either by a specific albumin receptor or by a nonspecific interaction between albumin and the cell surface.…”

“…Indeed, these two possible mechanisms are not mutually exclusive. Both may contribute, along with some form of nonspecific surface-mediated dissociation process (14,17,71,73) to the patterns of uptake kinetics observed at unphysiologically low concentrations of albumin. Indeed, our studies cannot rule out the possibility that such a mechanism may also exist at physiologic albumin concentrations.…”

Uptake of oleate from albumin solutions by rat liver. Failure to detect catalysis of the dissociation of oleate from albumin by an albumin receptor.

“…Although this model can account for the original kinetic observations (1) and has enjoyed some success in predicting subsequent findings (9), an alternative model has recently been proposed that does not require postulating an interaction of albumin with the liver cell (10). This dissociation-limited model retains the traditional assumption that the hepatic removal mechanism acts only on free ligand.…”