Organization of functional groups of liver bilitranslocase.

Bilitranslocase transport activity can be described as consisting of three functional fractions, which depend on two distinct classes of sulfhydryl groups, on the one hand, and on the guanido groups of arginine residues, on the other. Each fraction accounts for approx. 50% transport activity. The pattern of transport activity inhibition resulting from step-wise derivatization of these functional groups indicates that, in general, derivatization of arginine residues prevents that of one class of sulfhydryl groups and vice versa, indicating their close location in the protein. Nevertheless, under appropriate conditions, derivatization of both functional groups can be achieved; however, the inhibitory effect produced is not additive. Hence, these two fractions overlap functionally and are likely to belong to a common functional domain of the protein. On the contrary, the other class of sulfhydryl groups can be derivatized, regardless of the state of the arginine residues.