Partial isolation and characterization of bromosulphosphthalein-binding protein from rat liver plasma membranes.

Digestion of liver plasma membranes with trypsin and chymotrypsin prevented specific binding of 35-S or 131-I-bromosulphophthalein (BSP) to these membranes, in contrast concomitant appearance of BSP binding protein was observed in a high-speed (100,000 x g. 1 hr) supernatant of the extracts. On the other hand, BSP binding capacity of receptors was hardly destroyed by digestion with high concentration of phospholipase A, C. Gel filtration experiments on a column of Sephadex indicated that specific BSP binding to a protein in the high-speed supernatant was observed and this protein contained at least sialic acid and pentose suggesting a fragment of glycoprotein. In addition this molecular size was far smaller than 23,000, calibrated with bovine trypsin. Competitive inhibition was also observed between 131-I-BSP and BSP on a specific protein by gel filtration, while cholic acid did not affect its capacity to bind 131I-BSP. These results suggest that the specific protein on the liver plasma membrane is involved in the transport of organic anions across hepatocyte surface membranes.