In mammals, active transport of organic solutes across plasma membranes was thought to be primarily driven by the Na+ gradient. Here we report the cloning and functional characterization of a H(+)-coupled transporter of oligopeptides and peptide-derived antibiotics from rabbit small intestine. This new protein, named PepT1, displays an unusually broad substrate specificity. PepT1-mediated uptake is electrogenic, independent of extracellular Na+, K+ and Cl-, and of membrane potential. PepT1 messenger RNA was found in intestine, kidney and liver and in small amounts in brain. In the intestine, the PepT1 pathway constitutes a major mechanism for absorption of the products of protein digestion. To our knowledge, the PepT1 primary structure is the first reported for a proton-coupled organic solute transporter in vertebrates and represents an interesting evolutionary link between prokaryotic H(+)-coupled and vertebrate Na(+)-coupled transporters of organic solutes.
We investigated the transport of cationic neurotoxins and neurotransmitters by the potential-sensitive organic transporter OCT3 and its steroid sensitivity using heterologous expression systems and also analyzed the expression of OCT3 in the brain. When expressed in mammalian cells, OCT3 mediates the uptake of the neurotoxin 1-methyl-4-phenylpyridinium (MPP ؉ ) and the neurotransmitter dopamine. Competition experiments show that several cationic neuroactive agents including amphetamines interact with OCT3. When expressed in Xenopus laevis oocytes, OCT3-mediated MPP ؉ uptake is associated with inward currents under voltage-clamp conditions. The MPP ؉ -induced currents are saturable with respect to MPP ؉ concentration, and half-maximal saturation (K 0.5 ) occurs at about 25 M MPP ؉ with membrane potential clamped at ؊50 mV. The K 0.5 for MPP ؉ is markedly influenced by membrane potential. OCT3 is inhibited by several steroids, and -estradiol is the most potent inhibitor (K i ϳ1 M). The pattern of steroid sensitivity of OCT3 is different from that of OCT1 and OCT2 but correlates significantly with that of the extraneuronal monoamine transporter (uptake 2 ). The transport characteristics and steroid sensitivity provide strong evidence for the molecular identity of OCT3 as uptake 2 . OCT3 is expressed in the brain as evidenced from Northern blot analysis, reverse transcription-polymerase chain reaction, and in situ hybridization using OCT3-specific probes. The molecular identity of the transcript hybridizing to the probe has been established by sequencing the reverse transcription-polymerase chain reaction product and also by the isolation of the OCT3 cDNA from a brain cDNA library. Regional distribution studies with in situ hybridization show that OCT3 is expressed widely in different brain regions, especially in the hippocampus, cerebellum, and cerebral cortex. OCT3 is likely to play a significant role in the disposition of cationic neurotoxins and neurotransmitters in the brain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.