Gastric uptake of nicotinic acid by bilitranslocase

Sabina Passamonti*, Lucia Battiston, Gian Luigi SottocasaBilitranslocase is a 38 kDa membrane protein [1] localised atthe vascular surface of the liver cells. The isolated protein isendowed with transport function of sulphobromophthalein(SBP), as directly demonstrated in reconstituted systems.Thus, its proposed physiological role is to mediate the tra⁄cof SBP and related organic anions across the sinusoidal do-main of the hepatocyte plasma membrane. On account of theknown competition between SBP and bilirubin for the ¢rststep of liver uptake observed in vivo, bilirubin had beenpointed out as the most obvious physiological substrate forbilitranslocase. A strong indication in this sense has come withthe ¢nding, within bilitranslocase primary structure, of thesequence EDSQGQHLSSF [2], which has the peculiarity ofbeing the central portion of a motif highly conserved in K-phycocyanins, biliproteins found in cyanobacteria. Whereasin K-phycocyanins this motif is involved in binding the pros-thetic group phycocyanobilin [3], a compound structurally re-lated to bilirubin, in bilitranslocase it has been shown to act asthe bilirubin-binding site [2]. Indeed, the a⁄nity of the carrierfor the pigment is high enough to ensure bilirubin extractionfrom the blood stream (K