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R A Weisiger

Publications and source records attributed to R A Weisiger.

61 records · Page 4Linked to original sources
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Effect of albumin binding on extraction of sulfobromophthalein by perfused elasmobranch liver: evidence for dissociation-limited uptake.

We investigated the role of serum albumin in the hepatic uptake of organic anions by determining the effect of added bovine albumin on sulfobromophthalein (BSP) uptake in skates, an animal which naturally lacks this protein. A single-pass perfused liver model was used to determine steady-state net uptake rates as the BSP or BSP and albumin concentrations were systematically varied. Results indicated that two different steps in the uptake process are capable of limiting the uptake rate depending on the albumin and BSP concentrations selected. At higher rates of uptake corresponding to higher BSP concentrations (up to 112 microM), saturation kinetics were observed as the BSP concentration was varied, with apparent Km and Vmax values which were independent of the albumin concentration (0.05 to 0.75%). These data suggest that under these conditions uptake is limited by a saturable step intrinsic to the liver. In contrast, for lower BSP concentrations and albumin concentrations below about 0.5%, a different kinetic pattern was seen which suggested that the rate-limiting step in uptake was transfer of BSP from albumin to the liver. The latter data were found to be consistent with a model in which the limiting step in the transfer process is spontaneous dissociation of the BSP from binding sites on albumin within the sinusoid. These results suggest that skate liver clears BSP from albumin solutions by a two-step mechanism in which dissociation from albumin is followed by a saturable process consistent with carrier-mediated transport. The albumin concentration and uptake velocity are important factors in determining which of these steps limits the uptake rate. Clearance is efficient and irreversible even though elasmobranch liver lacks high-affinity cytosolic binding proteins for BSP, and elasmobranch plasma does not normally contain albumin.

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Interaction of natural and synthetic albumin polymers with hepatocytes.

The hepatitis B virus binds avidly to albumin polymers which in turn may mediate the initial binding of viral particles to the liver cell. However, the interaction of albumin polymers with the liver remains poorly characterized, and the possibility that hepatic binding reflects an artifact of polymerization with glutaraldehyde has not been excluded. We therefore characterized the binding of 125I-labeled natural and synthetic albumin polymers to suspensions of rat hepatocytes. Saturable binding was demonstrated for all preparations of monomeric and polymeric albumin studied. Glutaraldehyde-polymerized albumin (mean polymerization number = 15) bound much more avidly than naturally occurring albumin polymers (mostly dimers and trimers) or monomeric albumin. Competition between monomer and synthetic polymer was not observed. Reduction of free aldehyde groups on the synthetic polymer decreased nonsaturable binding without affecting saturable binding. Autoradiography confirmed binding of polyalbumin to hepatic parenchymal cells. Glutaraldehyde-polymerized ovalbumin, a protein unrelated to serum albumin, also bound hepatocytes saturably. We conclude that hepatic binding of synthetic albumin polymers is not due to residual aldehyde groups on the polymer and is much more avid than for natural polymer. This difference may reflect the higher degree of polymerization or chemical modification of the synthetic polymer. The hepatic binding sites for synthetic polymer appear distinct from those previously described for monomeric albumin and may not be specific for albumin.

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