4-Hydroxyacetophenone-induced choleresis in rats is mediated by the Mrp2-dependent biliary secretion of its glucuronide conjugate.

PURPOSE: The present study examined the underlying mechanism by which 4-hydroxyacetophenone (4-HA), a bioactive compound found in several medicinal herbs, exerts its potent stimulatory effects on hepatic bile secretion. METHODS: Bile flow, and biliary excretion of 4-HA, its metabolites, and inorganic electrolytes was examined in both normal Wistar rats and in TR(-) Wistar rats that have a congenital defect in the multidrug resistance-associated protein-2, Mrp2/Abcc2. The effects of 4-HA were also examined in animals treated with buthionine sulfoximine to decrease hepatic glutathione (GSH) levels. RESULTS: In normal rats, 4-HA dramatically increased bile flow rate, whereas it failed to exert a choleretic effect in TR(-) rats. This choleresis was not explained by increased biliary output of Na(+), K(+), Cl(-) or HCO(3) (-), or by increased biliary GSH excretion. Depletion of hepatic GSH with buthionine sulfoximine had no effect on the 4-HA-induced choleresis. HPLC analysis revealed that a single major compound was present in bile, namely.4-hydroxyacetophenone-4-O-beta-glucuronide, and that the parent compound was not detected in bile. Biliary excretion of the glucuronide was directly correlated with the increases in bile flow. In contrast to normal rats, this 4-HA metabolite was not present in bile of TR(-) rats. CONCLUSIONS: These results demonstrate that the major biliary metabolite of 4-HA in rats is the 4-O-beta-glucuronide, a compound that is secreted into bile at high concentrations, and may thus account in large part for the choleretic effects of 4-HA. Transport of this metabolite across the canalicular membrane into bile requires expression of the Mrp2 transport protein.

Transport of a neurotoxicant by molecular mimicry: the methylmercury-L-cysteine complex is a substrate for human L-type large neutral amino acid transporter (LAT) 1 and LAT2.

Methylmercury (MeHg) readily crosses cell membrane barriers to reach its target tissue, the brain. Although it is generally assumed that this rapid transport is due to simple diffusion, recent studies have demonstrated that MeHg is transported as a hydrophilic complex, and possibly as an L-cysteine complex on the ubiquitous L-type large neutral amino acid transporters (LATs). To test this hypothesis, studies were carried out in Xenopus laevis oocytes expressing two of the major L-type carriers in humans, LAT1-4F2 heavy chain (4F2hc) and LAT2-4F2hc. Oocytes expressing LAT1-4F2hc or LAT2-4F2hc demonstrated enhanced uptake of [(14)C]MeHg when administered as the L-cysteine or D,L-homocysteine complexes, but not when administered as the D-cysteine, N -acetyl-L-cysteine, penicillamine or GSH complexes. Kinetic analysis of transport indicated that the apparent affinities ( K (m)) of MeHg-L-cysteine uptake by LAT1 and LAT2 (98+/-8 and 64+/-8 microM respectively) were comparable with those for methionine (99+/-9 and 161+/-11 microM), whereas the V (max) values were higher for MeHg-L-cysteine, indicating that it may be a better substrate than the endogenous amino acid. Uptake and efflux of [(3)H]methionine and [(14)C]MeHg-L-cysteine were trans -stimulated by leucine and phenylalanine, but not by glutamate, indicating that MeHg-L-cysteine is both a cis - and trans -substrate. In addition, [(3)H]methionine efflux was trans -stimulated by leucine and phenylalanine even in the presence of an inwardly directed methionine gradient, demonstrating concentrative transport by both LAT1 and LAT2. The present results describe a major molecular mechanism by which MeHg is transported across cell membranes and indicate that metal complexes may form a novel class of substrates for amino acid carriers. These transport proteins may therefore participate in metal ion homoeostasis and toxicity.