The quinoid structure is the molecular requirement for recognition of phthaleins by the organic anion carrier at the sinusoidal plasma membrane level in the liver.

Sulfobromophthalein electrogenic uptake into rat liver plasma membrane vesicles was shown to admit only the quinoid, trivalent anion. The minimum requirement for this electrogenic process has been investigated in rat liver plasma membrane vesicles by using Thymol blue, a pH-indicator phthalein occurring either as a neutral, phenolic molecule or as a quinoid, monovalent anion. It has been found that Thymol blue is taken up electrogenically, in accordance with Michaelis-Menten kinetics. Parallel inhibition experiments have shown that both sulfobromophthalein and Thymol blue electrogenic uptakes are performed by the same carrier. It is, therefore, concluded that the phthalein structure recognized for transport is the quinoid molecule, with the dissociated acidic function on the benzene ring. Moreover, inhibitions by rifamycin-SV and bilirubin suggest that there exists a common uptake system for bilirubin, phthaleins and other anions. Taurocholate, on the contrary, does not appear to be involved in the same process.