Sulfobromophthalein/Bilirubin Binding Protein

To clarify sulfobromophthalein (BSP) and bilirubin uptake mechanisms, isolated rat hepatocytes were incubated with [5SJBSP. The initial uptake velocity (V.), determined from the first, linear portion of the cumulative uptake curve, was saturable (Michaelis constant [K.i = 6.2±0.5 ;sM; V.,= = 638±33 pmol X minl' per 10i hepatocytes), maximal at 370C and pH 7.4, and competitively inhibited by bilirubin, but not by taurocholate, cholate, or oleate. Preloading with unlabeled BSP led to transstimulation of V.. Sodium substitution or pretreatment of hepatocytes with ouabain or metabolic inhibitors had no effect on V.; trypsin reduced V. by 39% (P < 0.001). A rabbit antiserum to the rat liver plasma membrane (LPM)-BSP/bilirubin binding protein selectively reduced V. of5 ;&M I35SIBSP and I'4Clbilirubin by 41 and 42%, respectively (P < 0.01); uptakes of I3Hloleate, I3Hlcholate and [3EHtaurocholate were not affected. Hence, the LPM-BSP/bilirubin binding protein plays a role in the carriermediated uptake of BSP and bilirubin by hepatocytes.