AJP - Gastrointestinal and Liver Physiology, Vol 263, Issue 3 338-G344, Copyright © 1992 by American Physiological Society
Carrier-mediated transport of tetrabromosulfonephthalein by rat liver plasma membrane vesicles
A. M. Torres, J. V. Rodriguez, G. C. Lunazzi and C. Tiribelli
Centro Studi Fegato, Universita di Trieste, Italy.
To investigate the molecular requirements and mechanisms for the hepatic
uptake of phthaleins, the transport of tetrabromosulfonephthalein (TBS) was
investigated in basolateral rat liver plasma membrane vesicles. TBS uptake
was electrogenic as greatly accelerated by the creation of a
positive-inside membrane potential by the addition of valinomycin in the
presence of an inwardly directed potassium gradient. No effect was observed
when the ionophore was added in the presence of a sodium gradient. The
transport occurred into an osmotic-sensitive space and was saturable with
an apparent Michaelis constant of 5.32 +/- 0.56 microM and a maximal
velocity of 9.23 +/- 0.25 nmol.s-1.mg protein-1 (mean +/- SD, n = 3
experiments). TBS uptake was directly related to the extra-vesicular pH,
indicating the deprotonated quinoid negative-charged form of the dye as the
transported species. In contrast, TBS uptake was inversely related to the
intravesicular pH, suggesting that protonation inside the vesicles may act
as an efficient trap in transport process. Addition of polyclonal
monospecific anti-bilitranslocase antibody to liver vesicles specifically
inhibited TBS uptake rate (3.27 +/- 0.17 vs. 5.82 +/- 0.61 nmol.s-1.mg
protein-1, n = 3, P less than 0.001). These data indicate that TBS is
electrogenically transported across the liver cell plasma membrane by
bilitranslocase. They also indicate that the presence of a negative charged
group on the benzenic ring of the ligand is important in accounting for the
transport.
