Urinary excretion accounts for two-thirds of total elimination of uric acid and the remainder is excreted in feces. However, the mechanism of extra-renal elimination is poorly understood. In the present study, we aimed to clarify the mechanism and the extent of elimination of uric acid through liver and intestine using oxonate-treated rats and Caco-2 cells as a model of human intestinal epithelium. In oxonate-treated rats, significant amounts of externally administered and endogenous uric acid were recovered in the intestinal lumen, while biliary excretion was minimal. Accordingly, direct intestinal secretion was thought to be a substantial contributor to extra-renal elimination of uric acid. Since human efflux transporter BCRP/ABCG2 accepts uric acid as a substrate and genetic polymorphism causing a decrease of BCRP activity is known to be associated with hyperuricemia and gout, the contribution of rBcrp to intestinal secretion was examined. rBcrp was confirmed to transport uric acid in a membrane vesicle study, and intestinal regional differences of expression of rBcrp mRNA were well correlated with uric acid secretory activity into the intestinal lumen. Bcrp1 knockout mice exhibited significantly decreased intestinal secretion and an increased plasma concentration of uric acid. Furthermore, a Bcrp inhibitor, elacridar, caused a decrease of intestinal secretion of uric acid. In Caco-2 cells, uric acid showed a polarized flux from the basolateral to apical side, and this flux was almost abolished in the presence of elacridar. These results demonstrate that BCRP contributes at least in part to the intestinal excretion of uric acid as extra-renal elimination pathway in humans and rats.

Previous studies have shown that a Na⁺-dependent transport system is responsible for the transplacental transfer of the vitamins pantothenate and biotin and the essential metabolite lipoate. We now report the isolation of a rat placental cDNA encoding a transport protein responsible for this function. The cloned cDNA, when expressed in HeLa cells, induces Na⁺-dependent pantothenate and biotin transport activities. The transporter is specific for pantothenate, biotin, and lipoate. The Michaelis-Menten constant (K _t) for the transport of pantothenate and biotin in cDNA-transfected cells is 4.9 ± 1.1 and 15.1 ± 1.2 μm, respectively. The transport of both vitamins in cDNA-transfected cells is inhibited by lipoate with an inhibition constant (K _i) of approximately 5 μm. The nucleotide sequence of the cDNA (sodium-dependent multivitamin transporter (SMVT)) predicts a protein of 68.6 kDa with 634 amino acids and 12 potential transmembrane domains. Protein data base search indicates significant sequence similarity between SMVT and known members of the Na⁺-dependent glucose transporter family. Northern blot analysis shows that SMVT transcripts are present in all of the tissues that were tested. The size of the principal transcript is 3.2 kilobases. SMVT represents the first Na⁺-dependent vitamin transporter to be cloned from a mammalian tissue.

σ receptor is a protein that interacts with a variety of psychotomimetic drugs including cocaine and amphetamines and is believed to play an important role in the cellular functions of various tissues associated with the endocrine, immune, and nervous systems. Here we report on the structure and organization of the human gene coding for this receptor. The gene is ∼7 kbp long and contains four exons, interrupted by three introns. Exon 3 is the shortest (93 bp), and exon 4 is the longest (1,132 bp). Among the introns, intron 3 is the longest (∼1,250 bp). Exon 2 codes for the single transmembrane domain present in the receptor. 5′ rapid amplification of cDNA end reactions with mRNA from the JAR human trophoblast cell line have identified 56 bp upstream of the translation start codon as the initiation site for transcription. This transcription start site has been confirmed by RNase protection analysis. Structural analysis of the 5′ flanking region has revealed that the gene is TATA‐less. This region, however, contains a CCAATC box in the reverse complement and several GC boxes that are recognition sites for SP1. There are also consensus sequences for the liver‐specific transcription factor nuclear factor‐1/L, for a variety of cytokine responsive factors, and for the xenobiotic responsive factor called the arylhydrocarbon receptor. Southern blot analysis of the genomic DNA from Chinese hamster‐human and mouse‐human hybrid cell lines and fluorescent in situ hybridization with human metaphase chromosome spreads have shown that the gene is located on human chromosome 9, band p13, a region known to be associated with different psychiatric disorders.

BACKGROUND AND PURPOSEAn ATP‐binding cassette (ABC) transporter, breast cancer resistance protein (BCRP)/ABCG2, limits oral bioavailability of sulphasalazine. Here we examined the effect of curcumin, the principal curcuminoid of turmeric, on oral bioavailability of microdoses and therapeutic doses of sulphasalazine in humans.EXPERIMENTAL APPROACHEffects of curcumin were measured on the ATP‐dependent sulphasalazine uptake by hBCRP‐expressing membrane vesicles and on oral bioavailability of sulphasalazine in wild‐type and Bcrp(–/–) mice. Eight healthy Japanese subjects received an oral dose of sulphasalazine suspension (100 µg) or tablets (2 g) alone or after curcumin tablets (2 g). Uptake of sulphasalazine was studied in HEK293 cells transfected with the influx transporter (OATP)2B1.KEY RESULTSCurcumin was a potent hBCRP inhibitorin vitro(K_i0.70 ± 0.41 µM). Curcumin increased the area under the curve (AUC)_0–8of plasma sulphasalazine eightfold in wild‐type mice at 300 and 400 mg·kg⁻¹, but not in Bcrp(–/–) mice. Curcumin increased AUC_0–24of plasma sulphasalazine 2.0‐fold at microdoses and 3.2‐fold at therapeutic doses in humans. Non‐linearity of the dose–exposure relationship was observed between microdoses and therapeutic doses of sulphasalazine. Sulphasalazine was a substrate for OATP2B1 (K_m1.7 ± 0.3 µM). Its linear index (dose/K_m) at the therapeutic dose was high and may saturate OATP2B1.CONCLUSIONS AND IMPLICATIONSCurcumin can be used to investigate effects of BCRP on oral bioavailability of drugs in humans. Besides the limited dissolution, OATP2B1 saturation is a possible mechanism underlying non‐linearity in the dose–exposure relationship of sulphasalazine.