Plasma clearance of sulfobromophthalein and its interaction with hepatic binding proteins in normal and analbuminemic rats: is plasma albumin essential for vectorial transport of organic anions in the liver?

Abstract

To investigate a possible function of plasma albumin in the vectorial transport of organic anions by the liver, the plasma disappearance of sulfobromophthalein (BSP) and its interaction with plasma and liver cytosolic proteins were studied in normal rats and mutant Nagase analbuminemic rats (NAR). After intravenous administration of BSP, plasma BSP decreased rapidly in both NAR and control animals: plasma clearance values of BSP in NAR and controls were 12.45 and 7.40 ml/min per kg, respectively. Gel exclusion Sephadex G-100 chromatography of BSP with control rat serum revealed a protein peak in the void volume and another in the albumin fraction. BSP chromatographed exclusively with the albumin fraction; binding of BSP to plasma albumin occurred stoichiometrically. Similar studies with NAR serum revealed a single protein peak, in the void volume; a small amount of BSP chromatographed with this protein peak. The amount of BSP that chromatographed with NAR serum protein(s) was 8% of that with control rat serum albumin. Sephadex G-100 chromatography of BSP with control rat liver cytosol revealed four peaks of protein-bound BSP in fractions corresponding to the void volume (fraction X), albumin, glutathione S-transferases (fraction Y, Mr 45,000), and fraction Z (Mr 12,000); fraction Y was the major component of BSP binding. Gel chromatography of NAR liver cytosol with BSP revealed three BSP peaks, fractions X, Y, and Z; fraction X was the major component of BSP binding. Total BSP binding by 30 mg of hepatic cytosolic proteins was 4.5 nmol for controls and 10.4 nmol for NAR. Isoelectric focusing of liver cytosol revealed no quantitative or qualitative differences in glutathione S-transferase isozymes between control and mutant animals. Intravenously administered BSP (5 mumol/kg) rapidly appeared in bile as the free form and the glutathione conjugate in normal rats and NAR; 41% and 57% of injected BSP was excreted within 60 min in NAR and control rat bile, respectively. These results indicate that binding of BSP to plasma albumin is not indispensable to transhepatocyte transport of BSP in vivo.