Arylsulfonylation of bilitranslocase in plasma membranes from rat liver enables to discriminate between natural and artificial substrates.

The serine protease inhibitor phenylmethylsulfonyl fluoride is shown to cause partial inhibition of bilitranslocase transport activity in rat liver plasma membrane vesicles. This condition can be fully reversed by means of pyridine-2-al-doxime methiodide, indicating that the carrier has undergone sulfonylation. Protection against inactivation is afforded by both bilirubin, the natural substrate, and nicotinic acid, but, unexpectedly, by neither sulfobromophthalein, the chromophoric substrate employed in bilitranslocase transport activity assay, nor rifamycin SV, a competitive inhibitor of sulfobromophthalein transport. From these protection experiments, the Ka for the complex of bilitranslocase with either bilirubin or nicotinic acid has been estimated to be 2.1 and 10.8 nM. respectively. Tentatively, the target for phenylmethylsulfonyl fluoride on bilitranslocase is identified as a recognition site for the physiological substrates.