Characterization of electrogenic bromosulfophthalein transport in carnation petal microsomes and its inhibition by antibodies against bilitranslocase

Bilitranslocase is a rat liver plasma membrane carrier, displaying a high-affinity binding site for bilirubin. It is competitively inhibited by grape anthocyanins, including aglycones and their mono- and di-glycosylated derivatives. In plant cells, anthocyanins are synthesized in the cytoplasm and then translocated into the central vacuole, by mechanisms yet to be fully characterized. The aim of this work was to determine whether a homologue of rat liver bilitranslocase is expressed in carnation petals, where it might play a role in the membrane transport of anthocyanins. The bromosulfophthalein-based assay of rat liver bilitranslocase transport activity was implemented in subcellular membrane fractions, leading to the identification of a bromosulfophthalein carrier (K-M = 5.3 mu M), which is competitively inhibited by cyanidine 3-glucoside (K-i = 51.6 mu M) and mainly noncompetitively by cyanidin (K-i = 88.3 mu M). Two antisequence antibodies against bilitranslocase inhibited this carrier. In analogy to liver bilitranslocase, one antibody identified a bilirubin-binding site (K-d = 1.7 nM) in the carnation carrier. The other antibody identified a high-affinity binding site for cyanidine 3-glucoside (K-d = 1.7 mu M) on the carnation carrier only, and a high-affinity bilirubin-binding site (K-d = 0.33 nm) on the liver carrier only. Immunoblots showed a putative homologue of rat liver bilitranslocase in both plasma membrane and tonoplast fractions, isolated from carnation petals. Furthermore, only epidermal cells were immunolabelled in petal sections examined by microscopy. In conclusion, carnation petals express a homologue of rat liver bilitranslocase, with a putative function in the membrane transport of secondary metabolites.