Bilitranslocase, a plasma membrane protein involved in bilirubin and other organic anion uptake by the liver, exhibits a high molecular weight (170 000) when isolated in the presence of deoxycholate. This value is decreased to approx. 100 000 if deoxycholate is not included in the isolation medium. Both preparations can be resolved into two kinds of subunit, alpha and beta , of 37 000 and 35 500, respectively, by reduction with 2-mercaptoethanol and addition of sodium dodecyl sulfate. Under these conditions the two subunits are still capable of high-affinity sulfobromophthalein binding and, despite the presence of the detergent, may be isolated by preparative polyacrylamide gel electrophoresis still associated with the dye. It may be suggested that the physiological subunit composition of bilitranslocase is alpha 2-beta.
